Exim 4.40 Specification chapter 14

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# 14. Main configuration

The first part of the run time configuration file contains three types of item:

- Macro definitions: These lines start with an upper case letter. See section 6.4 for details of macro processing.

- Named list definitions: These lines start with one of the words “domainlist”, “hostlist”, “addresslist”, or “localpartlist”. Their use is described in section 10.5.

- Main configuration settings: Each setting occupies one line of the file (with possible continuations). If any setting is preceded by the word “hide”, the -bP command line option displays its value to admin users only. See section 6.6 for a description of the syntax of these option settings.

This chapter specifies all the main configuration options, along with their types and default values. For ease of finding a particular option, they appear in alphabetical order in section 14.23 below. However, because there are now so many options, they are first listed briefly in functional groups, as an aid to finding the name of the option you are looking for. Some options are listed in more than one group.

## 14.1. Miscellaneous

`bi_command` | to run for -bi command line option
`keep_malformed` | for broken files – should not happen
`localhost_number` | for unique message ids in clusters
`message_body_visible` | how much to show in `$message_body`
`print_topbitchars` | top-bit characters are printing
`timezone` | force time zone

## 14.2. Exim parameters

`exim_group` | override compiled-in value
`exim_path` | override compiled-in value
`exim_user` | override compiled-in value
`primary_hostname` | default from uname()
`split_spool_directory` | use multiple directories
`spool_directory` | override compiled-in value

## 14.3. Privilege controls

`admin_groups` | groups that are Exim admin users
`deliver_drop_privilege` | drop root for delivery processes
`local_from_check` | insert Sender: if necessary
`local_from_prefix` | for testing From: for local sender
`local_from_suffix` | for testing From: for local sender
`local_sender_retain` | keep Sender: from untrusted user
`never_users` | do not run deliveries as these
`prod_requires_admin` | forced delivery requires admin user
`queue_list_requires_admin` | queue listing requires admin user
`trusted_groups` | groups that are trusted
`trusted_users` | users that are trusted

## 14.4. Logging

`log_file_path` | override compiled-in value
`log_selector` | set/unset optional logging
`log_timezone` | add timezone to log lines
`message_logs` | create per-message logs
`preserve_message_logs` | in another directory after message completion
`process_log_path` | for SIGUSR1 and exiwhat
`syslog_duplication` | controls duplicate log lines on syslog
`syslog_facility` | set syslog “facility” field
`syslog_processname` | set syslog “ident” field
`syslog_timestamp` | timestamp syslog lines
`write_rejectlog` | control use of message log

## 14.5. Frozen messages

`auto_thaw` | sets time for retrying frozen messages
`freeze_tell` | send message when freezing
`move_frozen_messages` | to another directory
`timeout_frozen_after` | keep frozen messages only so long

## 14.6. Data lookups

`ldap_default_servers` | used if no server in query
`ldap_version` | set protocol version
`lookup_open_max` | lookup files held open
`mysql_servers` | as it says
`oracle_servers` | as it says
`pgsql_servers` | as it says

## 14.7. Message ids

`message_id_header_domain` | used to build Message-ID: header
`message_id_header_text` | ditto

## 14.8. Embedded Perl Startup

`perl_at_start` | always start the interpreter
`perl_startup` | code to obey when starting Perl

## 14.9. Daemon

`daemon_smtp_ports` | default ports
`extra_local_interfaces` | not necessarily listened on
`local_interfaces` | on which to listen, with optional ports
`pid_file_path` | override compiled-in value
`queue_run_max` | maximum number of simultaneous queue runners

## 14.10. Resource control

`check_log_inodes` | before accepting a message
`check_log_space` | before accepting a message
`check_spool_inodes` | before accepting a message
`check_spool_space` | before accepting a message
`deliver_queue_load_max` | no queue deliveries if load high
`queue_only_load` | queue incoming if load high
`queue_run_max` | maximum number of simultaneous queue runners
`remote_max_parallel` | parallel SMTP delivery per message
`smtp_accept_max` | simultaneous incoming connections
`smtp_accept_max_nommail` | non-mail commands
`smtp_accept_max_nonmail_hosts` | hosts to which the limit applies
`smtp_accept_max_per_connection` | messages per connection
`smtp_accept_max_per_host` | connections from one host
`smtp_accept_queue` | queue mail if more connections
`smtp_accept_queue_per_connection` | queue if more messages per connection
`smtp_accept_reserve` | only reserve hosts if more connections
`smtp_check_spool_space` | from SIZE on MAIL command
`smtp_connect_backlog` | passed to TCP/IP stack
`smtp_load_reserve` | SMTP from reserved hosts if load high
`smtp_reserve_hosts` | these are the reserve hosts

## 14.11. Policy controls

`acl_not_smtp` | set ACL for non-SMTP messages
`acl_smtp_auth` | set ACL for AUTH
`acl_smtp_connect` | set ACL for connection
`acl_smtp_data` | set ACL for DATA
`acl_smtp_etrn` | set ACL for ETRN
`acl_smtp_expn` | set ACL for EXPN
`acl_smtp_helo` | set ACL for EHLO or HELO
`acl_smtp_mail` | set ACL for MAIL
`acl_smtp_mailauth` | set ACL for AUTH on MAIL command
`acl_smtp_rcpt` | set ACL for RCPT
`acl_smtp_starttls` | set ACL for STARTTLS
`acl_smtp_vrfy` | set ACL for VRFY
`header_maxsize` | total size of message header
`header_line_maxsize` | individual header line limit
`helo_accept_junk_hosts` | allow syntactic junk from these hosts
`helo_allow_chars` | allow illegal chars in HELO names
`helo_lookup_domains` | lookup hostname for these HELO names
`helo_try_verify_hosts` | HELO soft-checked for these hosts
`helo_verify_hosts` | HELO hard-checked for these hosts
`host_lookup` | host name looked up for these hosts
`host_lookup_order` | order of DNS and local name lookups
`host_reject_connection` | reject connection from these hosts
`hosts_treat_as_local` | useful in some cluster configurations
`local_scan_timeout` | timeout for local_scan()
`message_size_limit` | for all messages
`percent_hack_domains` | recognize %-hack for these domains

## 14.12. Callout cache

`callout_domain_negative_expire` | timeout for negative domain cache item
`callout_domain_positive_expire` | timeout for positive domain cache item
`callout_negative_expire` | timeout for negative address cache item
`callout_positive_expire` | timeout for positive address cache item
`callout_random_local_part` | string to use for “random” testing

## 14.13. TLS

`tls_advertise_hosts` | advertise TLS to these hosts
`tls_certificate` | location of server certificate
`tls_crl` | certificate revocation list
`tls_dhparam` | DH parameters for server
`tls_privatekey` | location of server private key
`tls_remember_esmtp` | don't reset after starting TLS
`tls_require_ciphers` | specify acceptable cipers
`tls_try_verify_hosts` | try to verify client certificate
`tls_verify_certificates` | expected client certificates
`tls_verify_hosts` | insist on client certificate verify

## 14.14. Local user handling

`finduser_retries` | useful in NIS environments
`gecos_name` | used when creating Sender:
`gecos_pattern` | ditto
`max_username_length` | for systems that truncate
`unknown_login` | used when no login name found
`unknown_username` | ditto
`uucp_from_pattern` | for recognizing “From ” lines
`uucp_from_sender` | ditto

## 14.15. All incoming messages (SMTP and non-SMTP)

`header_maxsize` | total size of message header
`header_line_maxsize` | individual header line limit
`message_size_limit` | applies to all messages
`percent_hack_domains` | recognize %-hack for these domains
`received_header_text` | expanded to make Received:
`received_headers_max` | for mail loop detection
`recipients_max` | limit per message
`recipients_max_reject` | permanently reject excess

## 14.16. Non-SMTP incoming messages

`receive_timeout` | for non-SMTP messages

## 14.17. Incoming SMTP messages

See also the Policy controls section above.

`host_lookup` | host name looked up for these hosts
`host_lookup_order` | order of DNS and local name lookups
`recipient_unqualified_hosts` | may send unqualified recipients
`rfc1413_hosts` | make ident calls to these hosts
`rfc1413_query_timeout` | zero disables ident calls
`sender_unqualified_hosts` | may send unqualified senders
`smtp_accept_keepalive` | some TCP/IP magic
`smtp_accept_max` | simultaneous incoming connections
`smtp_accept_max_nommail` | non-mail commands
`smtp_accept_max_nonmail_hosts` | hosts to which the limit applies
`smtp_accept_max_per_connection` | messages per connection
`smtp_accept_max_per_host` | connections from one host
`smtp_accept_queue` | queue mail if more connections
`smtp_accept_queue_per_connection` | queue if more messages per connection
`smtp_accept_reserve` | only reserve hosts if more connections
`smtp_active_hostname` | host name to use in messages
`smtp_banner` | text for welcome banner
`smtp_check_spool_space` | from SIZE on MAIL command
`smtp_connect_backlog` | passed to TCP/IP stack
`smtp_enforce_sync` | of SMTP command/responses
`smtp_etrn_command` | what to run for ETRN
`smtp_etrn_serialize` | only one at once
`smtp_load_reserve` | only reserve hosts if this load
`smtp_max_unknown_commands` | before dropping connection
`smtp_ratelimit_hosts` | apply ratelimiting to these hosts
`smtp_ratelimit_mail` | ratelimit for MAIL commands
`smtp_ratelimit_rcpt` | ratelimit for RCPT commands
`smtp_receive_timeout` | per command or data line
`smtp_reserve_hosts` | these are the reserve hosts
`smtp_return_error_details` | give detail on rejections

## 14.18. SMTP extensions

`accept_8bitmime` | advertise 8BITMIME
`auth_advertise_hosts` | advertise AUTH to these hosts
`ignore_fromline_hosts` | allow “From ” from these hosts
`ignore_fromline_local` | allow “From ” from local SMTP
`pipelining_advertise_hosts` | advertise pipelining to these hosts
`tls_advertise_hosts` | advertise TLS to these hosts

## 14.19. Processing messages

`allow_domain_literals` | recognize domain literal syntax
`allow_mx_to_ip` | allow MX to point to IP address
`allow_utf8_domains` | in addresses
`delivery_date_remove` | from incoming messages
`envelope_to_remote` | from incoming messages
`extract_addresses_remove_arguments` | affects -t processing
`headers_charset` | default for translations
`qualify_domain` | default for senders
`qualify_recipient` | default for recipients
`return_path_remove` | from incoming messages
`strip_excess_angle_brackets` | in addresses
`strip_trailing_dot` | at end of addresses
`untrusted_set_sender` | untrusted can set envelope sender

## 14.20. System filter

`system_filter` | locate system filter
`system_filter_directory_transport` | transport for delivery to a directory
`system_filter_file_transport` | transport for delivery to a file
`system_filter_group` | group for filter running
`system_filter_pipe_transport` | transport for delivery to a pipe
`system_filter_reply_transport` | transport for autoreply delivery
`system_filter_user` | user for filter running

## 14.21. Routing and delivery

`dns_again_means_nonexist` | for broken domains
`dns_check_names_pattern` | pre-DNS syntax check
`dns_ipv4_lookup` | only v4 lookup for these domains
`dns_retrans` | parameter for resolver
`dns_retry` | parameter for resolver
`hold_domains` | hold delivery for these domains
`local_interfaces` | for routing checks
`queue_domains` | no immediate delivery for these
`queue_only` | no immediate delivery at all
`queue_only_file` | no immediate deliveryif file exists
`queue_only_load` | no immediate delivery if load is high
`queue_only_override` | allow command line to override
`queue_run_in_order` | order of arrival
`queue_run_max` | of simultaneous queue runners
`queue_smtp_domains` | no immediate SMTP delivery for these
`remote_max_parallel` | parallel SMTP delivery (per message, not overall)
`remote_sort_domains` | order of remote deliveries
`retry_data_expire` | timeout for retry data
`retry_interval_max` | safety net for retry rules

## 14.22. Bounce and warning messages

`bounce_message_file` | content of bounce
`bounce_message_text` | content of bounce
`bounce_return_body` | include body if returning message
`bounce_return_message` | include original message in bounce
`bounce_return_size_limit` | limit on returned message
`bounce_sender_authentication` | send authenticated sender with bounce
`errors_copy` | copy bounce messages
`errors_reply_to` | Reply-to: in bounces
`delay_warning` | time schedule
`delay_warning_condition` | condition for warning messages
`ignore_bounce_errors_after` | discard undeliverable bounces
`warn_message_file` | content of warning message

## 14.23. Alphabetical list of main options

### accept_8bitmime

Type: boolean
Default: false

This option causes Exim to send 8BITMIME in its response to an SMTP EHLO command, and to accept the BODY= parameter on MAIL commands. However, though Exim is 8-bit clean, it is not a protocol converter, and it takes no steps to do anything special with messages received by this route. Consequently, this option is turned off by default.

### acl_not_smtp

Type: string, expanded
Default: unset

This option defines the ACL that is run when a non-SMTP message is on the point of being accepted. See chapter 38 for further details.

### acl_smtp_connect

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP connection is received. See chapter 38 for further details.

### acl_smtp_auth

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP AUTH command is received. See chapter 38 for further details.

### acl_smtp_data

Type: string, expanded
Default: unset

This option defines the ACL that is run after an SMTP DATA command has been processed and the message itself has been received, but before the final acknowledgement is sent. See chapter 38 for further details.

### acl_smtp_etrn

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP ETRN command is received. See chapter 38 for further details.

### acl_smtp_expn

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP EXPN command is received. See chapter 38 for further details.

### acl_smtp_helo

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP EHLO or HELO command is received. See chapter 38 for further details.

### acl_smtp_mail

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP MAIL command is received. See chapter 38 for further details.

### acl_smtp_mailauth

Type: string, expanded
Default: unset

This option defines the ACL that is run when there is an AUTH parameter on a MAIL command. See chapter 38 for details of ACLs, and chapter 33 for details of authentication.

### acl_smtp_rcpt

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP RCPT command is received. See chapter 38 for further details.

### acl_smtp_starttls

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP STARTTLS command is received. See chapter 38 for further details.

### acl_smtp_vrfy

Type: string, expanded
Default: unset

This option defines the ACL that is run when an SMTP VRFY command is received. See chapter 38 for further details.

### admin_groups

Type: string list
Default: unset

If the current group or any of the supplementary groups of the caller is in this colon-separated list, the caller has admin privileges. If all your system programmers are in a specific group, for example, you can give them all Exim admin privileges by putting that group in `admin_groups`. However, this does not permit them to read Exim's spool files (whose group owner is the Exim gid). To permit this, you have to add individuals to the Exim group.

### allow_domain_literals

Type: boolean
Default: false

If this option is set, the RFC 2822 domain literal format is permitted in email addresses. The option is not set by default, because the domain literal format is not normally required these days, and few people know about it. It has, however, been exploited by mail abusers.

Unfortunately, it seems that some DNS black list maintainers are using this format to report black listing to postmasters. If you want to accept messages addressed to your hosts by IP address, you need to set `allow_domain_literals` true, and also to add `@[]` to the list of local domains (defined in the named domain list `local_domains` in the default configuration). This “magic string” matches the domain literal form of all the local host's IP addresses.

### allow_mx_to_ip

Type: boolean
Default: false

It appears that more and more DNS zone administrators are breaking the rules and putting domain names that look like IP addresses on the right hand side of MX records. Exim follows the rules and rejects this, giving an error message that explains the mis-configuration. However, some other MTAs support this practice, so to avoid “Why can”t Exim do this?' complaints, `allow_mx_to_ip` exists, in order to enable this heinous activity. It is not recommended, except when you have no other choice.

### allow_utf8_domains

Type: boolean
Default: false

Lots of discussion is going on about internationalized domain names. One camp is strongly in favour of just using UTF-8 characters, and it seems that at least two other MTAs permit this. This option allows Exim users to experiment if they wish.

If it is set true, Exim's domain parsing function allows valid UTF-8 multicharacters to appear in domain name components, in addition to letters, digits, and hyphens. However, just setting this option is not enough; if you want to look up these domain names in the DNS, you must also adjust the value of `dns_check_names_pattern` to match the extended form. A suitable setting is:

      dns_check_names_pattern = (?i)^(?>(?(1)\.|())[a-z0-9\xc0-\xff]\
        (?>[-a-z0-9\x80-\xff]*[a-z0-9\x80-\xbf])?)+$

Alternatively, you can just disable this feature by setting

      dns_check_names_pattern =

That is, set the option to an empty string so that no check is done.

### auth_advertise_hosts

Type: host list, expanded
Default: *

If any server authentication mechanisms are configured, Exim advertises them in response to an EHLO command only if the calling host matches this list. Otherwise, Exim does not advertise AUTH. Exim does not accept AUTH commands from clients to which it has not advertised the availability of AUTH. The advertising of individual authentication mechanisms can be controlled by the use of the `server_advertise_condition` generic authenticator option on the individual authenticators. See chapter 33 for further details.

Certain mail clients (for example, Netscape) require the user to provide a name and password for authentication if AUTH is advertised, even though it may not be needed (the host may accept messages from hosts on its local LAN without authentication, for example). The `auth_advertise_hosts` option can be used to make these clients more friendly by excluding them from the set of hosts to which Exim advertises AUTH.

If you want to advertise the availability of AUTH only when the connection is encrypted using TLS, you can make use of the fact that the value of this option is expanded, with a setting like this:

      auth_advertise_hosts = ${if eq{$tls_cipher}{}{}{*}}

If `$tls_cipher` is empty, the session is not encrypted, and the result of the expansion is empty, thus matching no hosts. Otherwise, the result of the expansion is *, which matches all hosts.

### auto_thaw

Type: time
Default: 0s

If this option is set to a time greater than zero, a queue runner will try a new delivery attempt on any frozen message if this much time has passed since it was frozen. This may result in the message being re-frozen if nothing has changed since the last attempt. It is a way of saying “keep on trying, even though there are big problems”. See also `timeout_frozen_after` and `ignore_bounce_errors_after`.

### bi_command

Type: string
Default: unset

This option supplies the name of a command that is run when Exim is called with the -bi option (see chapter 5). The string value is just the command name, it is not a complete command line. If an argument is required, it must come from the -oA command line option.

### bounce_message_file

Type: string
Default: unset

This option defines a template file containing paragraphs of text to be used for constructing bounce messages. Details of the file's contents are given in chapter 41. See also `warn_message_file`.

### bounce_message_text

Type: string
Default: unset

When this option is set, its contents are included in the default bounce message immediately after “This message was created automatically by mail delivery software.” It is not used if `bounce_message_file` is set.

### bounce_return_body

Type: boolean
Default: true

This option controls whether the body of an incoming message is included in a bounce message when `bounce_return_message` is true. If it is not set, only the message header is included.

### bounce_return_message

Type: boolean
Default: true

If this option is set false, the original message is not included in bounce messages generated by Exim. See also `bounce_return_size_limit`.

### bounce_return_size_limit

Type: integer
Default: 100K

This option sets a limit in bytes on the size of messages that are returned to senders as part of bounce messages when `bounce_return_message` is true. The limit should be less than the value of the global `message_size_limit` and of any `message_size_limit` settings on transports, to allow for the bounce text that Exim generates. If this option is set to zero there is no limit.

When the body of any message that is to be included in a bounce message is greater than the limit, it is truncated, and a comment pointing this out is added at the top. The actual cutoff may be greater than the value given, owing to the use of buffering for transferring the message in chunks (typically 8K in size). The idea is to save bandwidth on those undeliverable 15-megabyte messages.

### bounce_sender_authentication

Type: string
Default: unset

This option provides an authenticated sender address that is sent with any bounce messages generated by Exim that are sent over an authenticated SMTP connection. A typical setting might be:

      bounce_sender_authentication = mailer-daemon@my.domain.example

which would cause bounce messages to be sent using the SMTP command:

      MAIL FROM:<> AUTH=mailer-daemon@my.domain.example

The value of `bounce_sender_authentication` must always be a complete email address.

### callout_domain_negative_expire

Type: time
Default: 3h

This option specifies the expiry time for negative callout cache data for a domain. See section 38.21 for details of callout verification, and section 38.23 for details of the caching.

### callout_domain_positive_expire

Type: time
Default: 7d

This option specifies the expiry time for positive callout cache data for a domain. See section 38.21 for details of callout verification, and section 38.23 for details of the caching.

### callout_negative_expire

Type: time
Default: 2h

This option specifies the expiry time for negative callout cache data for an address. See section 38.21 for details of callout verification, and section 38.23 for details of the caching.

### callout_positive_expire

Type: time
Default: 24h

This option specifies the expiry time for positive callout cache data for an address. See section 38.21 for details of callout verification, and section 38.23 for details of the caching.

### callout_random_local_part

Type: string, expanded
Default: see below

This option defines the “random” local part that can be used as part of callout verification. The default value is

      $primary_host_name-$tod_epoch-testing

See section 38.22 for details of how this value is used.

### check_log_inodes

Type: integer
Default: 0

See `check_spool_space` below.

### check_log_space

Type: integer
Default: 0

See `check_spool_space` below.

### check_spool_inodes

Type: integer
Default: 0

See `check_spool_space` below.

### check_spool_space

Type: integer
Default: 0

The four `check_...` options allow for checking of disk resources before a message is accepted. `check_spool_space` and `check_spool_inodes` check the spool partition if either value is greater than zero, for example:

      check_spool_space = 10M
      check_spool_inodes = 100

The spool partition is the one which contains the directory defined by SPOOL_DIRECTORY in Local/Makefile. It is used for holding messages in transit.

`check_log_space` and `check_log_inodes` check the partition in which log files are written if either is greater than zero. These should be set only if `log_file_path` and `spool_directory` refer to different partitions.

If there is less space or fewer inodes than requested, Exim refuses to accept incoming mail. In the case of SMTP input this is done by giving a 452 temporary error response to the MAIL command. If ESMTP is in use and there was a SIZE parameter on the MAIL command, its value is added to the `check_spool_space` value, and the check is performed even if `check_spool_space` is zero, unless `no_smtp_check_spool_space` is set.

The values for `check_spool_space` and `check_log_space` are held as a number of kilobytes. If a non-multiple of 1024 is specified, it is rounded up.

For non-SMTP input and for batched SMTP input, the test is done at start-up; on failure a message is written to stderr and Exim exits with a non-zero code, as it obviously cannot send an error message of any kind.

### daemon_smtp_ports

Type: string
Default: `smtp`

This option specifies one or more default SMTP ports on which the Exim daemon listens. See chapter 13 for details of how it is used. For backward compatibility, `daemon_smtp_port` (singular) is a synonym.

### delay_warning

Type: time list
Default: 24h

When a message is delayed, Exim sends a warning message to the sender at intervals specified by this option. If it is set to a zero, no warnings are sent. The data is a colon-separated list of times after which to send warning messages. Up to 10 times may be given. If a message has been on the queue for longer than the last time, the last interval between the times is used to compute subsequent warning times. For example, with

      delay_warning = 4h:8h:24h

the first message is sent after 4 hours, the second after 8 hours, and the third one after 24 hours. After that, messages are sent every 16 hours, because that is the interval between the last two times on the list. If you set just one time, it specifies the repeat interval. For example, with:

      delay_warning = 6h

messages are repeated every six hours. To stop warnings after a given time, set a very large time at the end of the list. For example:

      delay_warning = 2h:12h:99d

### delay_warning_condition

Type: string, expanded
Default: see below

The string is expanded at the time a warning message might be sent. If all the deferred addresses have the same domain, it is set in `$domain` during the expansion. Otherwise `$domain` is empty. If the result of the expansion is a forced failure, an empty string, or a string matching any of “0”, “no” or “false” (the comparison being done caselessly) then the warning message is not sent. The default is

      delay_warning_condition = \
        ${if match{$h_precedence:}{(?i)bulk|list|junk}{no}{yes}}

which suppresses the sending of warnings about messages that have “bulk”, “list” or “junk” in a Precedence: header.

### deliver_drop_privilege

Type: boolean
Default: false

If this option is set true, Exim drops its root privilege at the start of a delivery process, and runs as the Exim user throughout. This severely restricts the kinds of local delivery that are possible, but is viable in certain types of configuration. There is a discussion about the use of root privilege in chapter 48.

### deliver_queue_load_max

Type: fixed-point
Default: unset

When this option is set, a queue run is abandoned if the system load average becomes greater than the value of the option. The option has no effect on ancient operating systems on which Exim cannot determine the load average. See also `queue_only_load` and `smtp_load_reserve`.

### delivery_date_remove

Type: boolean
Default: true

Exim's transports have an option for adding a Delivery-date: header to a message when it is delivered – in exactly the same way as Return-path: is handled. Delivery-date: records the actual time of delivery. Such headers should not be present in incoming messages, and this option causes them to be removed at the time the message is received, to avoid any problems that might occur when a delivered message is subsequently sent on to some other recipient.

### dns_again_means_nonexist

Type: domain list, expanded
Default: unset

DNS lookups give a “try again” response for the DNS errors “non-authoritative host not found” and “SERVERFAIL”. This can cause Exim to keep trying to deliver a message, or to give repeated temporary errors to incoming mail. Sometimes the effect is caused by a badly set up name server and may persist for a long time. If a domain which exhibits this problem matches anything in `dns_again_means_nonexist`, it is treated as if it did not exist. This option should be used with care. You can make it apply to reverse lookups by a setting such as this:

      dns_again_means_nonexist = *.in-addr.arpa

### dns_check_names_pattern

Type: string
Default: see below

When this option is set to a non-empty string, it causes Exim to check domain names for illegal characters before handing them to the DNS resolver, because some resolvers give temporary errors for malformed names. If a domain name contains any illegal characters, a “not found” result is forced, and the resolver is not called. The check is done by matching the domain name against a regular expression, which is the value of this option. The default pattern is

      dns_check_names_pattern = \
        (?i)^(?>(?(1)\.|())[^\W_](?>[a-z0-9-]*[^\W_])?)+$

which permits only letters, digits, and hyphens in components, but they may not start or end with a hyphen. If you set `allow_utf8_domains`, you must modify this pattern, or set the option to an empty string.

### dns_ipv4_lookup

Type: domain list, expanded
Default: unset

When Exim is compiled with IPv6 support, it looks for IPv6 address records (AAAA and, if configured, A6) as well as IPv4 address records when trying to find IP addresses for hosts, unless the host's domain matches this list.

This is a fudge to help with name servers that give big delays or otherwise do not work for the new IPv6 record types. If Exim is handed an IPv6 address record as a result of an MX lookup, it always recognizes it, and may as a result make an outgoing IPv6 connection. All this option does is to make Exim look only for IPv4-style A records when it needs to find an IP address for a host name. In due course, when the world's name servers have all been upgraded, there should be no need for this option.

### dns_retrans

Type: time
Default: 0s

The options `dns_retrans` and `dns_retry` can be used to set the retransmission and retry parameters for DNS lookups. Values of zero (the defaults) leave the system default settings unchanged. The first value is the time between retries, and the second is the number of retries. It isn't totally clear exactly how these settings affect the total time a DNS lookup may take. I haven't found any documentation about timeouts on DNS lookups; these parameter values are available in the external resolver interface structure, but nowhere does it seem to describe how they are used or what you might want to set in them.

### dns_retry

Type: integer
Default: 0

See `dns_retrans` above.

### drop_cr

Type: boolean
Default: false

This is an obsolete option that is now a no-op. It used to affect the way Exim handled CR and LF characters in incoming messages. What happens now is described in section 44.1.

### envelope_to_remove

Type: boolean
Default: true

Exim's transports have an option for adding an Envelope-to: header to a message when it is delivered – in exactly the same way as Return-path: is handled. Envelope-to: records the original recipient address from the messages's envelope that caused the delivery to happen. Such headers should not be present in incoming messages, and this option causes them to be removed at the time the message is received, to avoid any problems that might occur when a delivered message is subsequently sent on to some other recipient.

### errors_copy

Type: string list, expanded
Default: unset

Setting this option causes Exim to send bcc copies of bounce messages that it generates to other addresses. Note: this does not apply to bounce messages coming from elsewhere. The value of the option is a colon-separated list of items. Each item consists of a pattern, terminated by white space, followed by a comma-separated list of email addresses. If a pattern contains spaces, it must be enclosed in double quotes.

Each pattern is processed in the same way as a single item in an address list (see section 10.18). When a pattern matches the recipient of the bounce message, the message is copied to the addresses on the list. The items are scanned in order, and once a matching one is found, no further items are examined. For example:

      errors_copy = spqr@mydomain   postmaster@mydomain.example :\
                    rqps@mydomain   hostmaster@mydomain.example,\
                                    postmaster@mydomain.example

The address list is expanded before use. The expansion variables `$local_part` and `$domain` are set from the original recipient of the error message, and if there was any wildcard matching in the pattern, the expansion variables `$0`, `$1`, etc. are set in the normal way.

### errors_reply_to

Type: string
Default: unset

Exim's bounce and delivery warning messages contain the header line

`` `From: Mail Delivery System <Mailer-Daemon@<qualify-domain>>`

where <qualify-domain> is the value of the `qualify_domain` option. Experience shows that people reply to bounce messages. If the `errors_reply_to` option is set, a Reply-To: header is added to bounce and warning messages. For example:

      errors_reply_to = postmaster@my.domain.example

The value of the option is not expanded. It must specify a valid RFC 2822 address.

### exim_group

Type: string
Default: compile-time configured

This option changes the gid under which Exim runs when it gives up root privilege. The default value is compiled into the binary. The value of this option is used only when `exim_user` is also set. Unless it consists entirely of digits, the string is looked up using getgrnam(), and failure causes a configuration error. See chapter 48 for a discussion of security issues.

### exim_path

Type: string
Default: see below

This option specifies the path name of the Exim binary, which is used when Exim needs to re-exec itself. The default is set up to point to the file exim in the directory configured at compile time by the BIN_DIRECTORY setting. It is necessary to change `exim_path` if, exceptionally, Exim is run from some other place. Warning: Do not use a macro to define the value of this option, because you will break those Exim utilities that scan the configuration file to find where the binary is. (They then use the -bP option to extract option settings such as the value of `spool_directory`.)

### exim_user

Type: string
Default: compile-time configured

This option changes the uid under which Exim runs when it gives up root privilege. The default value is compiled into the binary. Ownership of the run time configuration file and the use of the -C and -D command line options is checked against the values in the binary, not what is set here.

Unless it consists entirely of digits, the string is looked up using getpwnam(), and failure causes a configuration error. If `exim_group` is not also supplied, the gid is taken from the result of getpwnam() if it is used. See chapter 48 for a discussion of security issues.

### extra_local_interfaces

Type: string list
Default: unset

This option defines network interfaces that are to be considered local when routing, but which are not used for listening by the daemon. See section 13.6 for details.

### extract_addresses_remove_arguments

Type: boolean
Default: true

According to some Sendmail documentation (Sun, IRIX, HP-UX), if any addresses are present on the command line when the -t option is used to build an envelope from a message's To:, Cc: and Bcc: headers, the command line addresses are removed from the recipients list. This is also how Smail behaves. However, other Sendmail documentation (the O'Reilly book) states that command line addresses are added to those obtained from the header lines. When `extract_addresses_remove_arguments` is true (the default), Exim subtracts argument headers. If it is set false, Exim adds rather than removes argument addresses.

### finduser_retries

Type: integer
Default: 0

On systems running NIS or other schemes in which user and group information is distributed from a remote system, there can be times when getpwnam() and related functions fail, even when given valid data, because things time out. Unfortunately these failures cannot be distinguished from genuine “not found” errors. If `finduser_retries` is set greater than zero, Exim will try that many extra times to find a user or a group, waiting for one second between retries.

### freeze_tell

Type: string list, comma separated
Default: unset

On encountering certain errors, or when configured to do so in a system filter, or in an ACL, Exim freezes a message. This means that no further delivery attempts take place until an administrator (or the `auto_thaw` feature) thaws the message. If `freeze_tell` is set, Exim generates a warning message whenever it freezes something, unless the message it is freezing is a locally-generated bounce message. (Without this exception there is the possibility of looping.) The warning message is sent to the addresses supplied as the comma-separated value of this option. If several of the message's addresses cause freezing, only a single message is sent. If the freezing was automatic, the reason(s) for freezing can be found in the message log. If you configure freezing in a filter or ACL, you must arrange for any logging that you require.

### gecos_name

Type: string, expanded
Default: unset

Some operating systems, notably HP-UX, use the “gecos” field in the system password file to hold other information in addition to users' real names. Exim looks up this field for use when it is creating Sender: or From: headers. If either `gecos_pattern` or `gecos_name` are unset, the contents of the field are used unchanged, except that, if an ampersand is encountered, it is replaced by the user's login name with the first character forced to upper case, since this is a convention that is observed on many systems.

When these options are set, `gecos_pattern` is treated as a regular expression that is to be applied to the field (again with & replaced by the login name), and if it matches, `gecos_name` is expanded and used as the user's name. Numeric variables such as `$1`, `$2`, etc. can be used in the expansion to pick up sub-fields that were matched by the pattern. In HP-UX, where the user's name terminates at the first comma, the following can be used:

      gecos_pattern = ([^,]*)
      gecos_name = $1

### gecos_pattern

Type: string
Default: unset

See `gecos_name` above.

### headers_charset

Type: string
Default: see below

This option sets a default character set for translating from encoded MIME “words” in header lines, when referenced by an `$h_xxx` expansion item. The default is the value of HEADERS_CHARSET in Local/Makefile. The ultimate default is ISO-8859-1. For more details see the description of header insertions in section 11.4.

### header_maxsize

Type: integer
Default: see below

This option controls the overall maximum size of a message's header section. The default is the value of HEADER_MAXSIZE in Local/Makefile; the default for that is 1M. Messages with larger header sections are rejected.

### header_line_maxsize

Type: integer
Default: 0

This option limits the length of any individual header line in a message, after all the continuations have been joined together. Messages with individual header lines that are longer than the limit are rejected. The default value of zero means “no limit”.

### helo_accept_junk_hosts

Type: host list, expanded
Default: unset

Exim checks the syntax of HELO and EHLO commands for incoming SMTP mail, and gives an error response for invalid data. Unfortunately, there are some SMTP clients that send syntactic junk. They can be accommodated by setting this option. Note that this is a syntax check only. See `helo_verify_hosts` if you want to do semantic checking. See also `helo_allow_chars` for a way of extending the permitted character set.

### helo_allow_chars

Type: string
Default: unset

This option can be set to a string of rogue characters that are permitted in all EHLO and HELO names in addition to the standard letters, digits, hyphens, and dots. If you really must allow underscores, you can set

      helo_allow_chars = _

Note that the value is one string, not a list.

### helo_lookup_domains

Type: domain list, expanded
Default: `@:@[]`

If the domain given by a client in a HELO or EHLO command matches this list, a reverse lookup is done in order to establish the host's true name. The default forces a lookup if the client host gives the server's name or any of its IP addresses (in brackets), something that broken clients have been seen to do.

### helo_try_verify_hosts

Type: host list, expanded
Default: unset

The RFCs mandate that a server must not reject a message because it doesn't like the HELO or EHLO command. By default, Exim just checks the syntax of these commands (see `helo_accept_junk_hosts` and `helo_allow_chars` above). However, some sites like to be stricter. If the calling host matches `helo_try_verify_hosts`, Exim checks that the host name given in the HELO or EHLO command either:

- is an IP literal matching the calling address of the host (the RFCs specifically allow this), or

- matches the host name that Exim obtains by doing a reverse lookup of the calling host address, or

- when looked up using gethostbyname() (or getipnodebyname() when available) yields the calling host address.

However, the EHLO or HELO command is not rejected if any of the checks fail. Processing continues, but the result of the check is remembered, and can be detected later in an ACL by the `verify = helo` condition. If you want verification failure to cause rejection of EHLO or HELO, use `helo_verify_hosts` instead.

### helo_verify_hosts

Type: host list, expanded
Default: unset

For hosts that match this option, Exim checks the host name given in the HELO or EHLO in the same way as for `helo_try_verify_hosts`. If the check fails, the HELO or EHLO command is rejected with a 550 error, and entries are written to the main and reject logs. If a MAIL command is received before EHLO or HELO, it is rejected with a 503 error.

### hold_domains

Type: domain list, expanded
Default: unset

This option allows mail for particular domains to be held on the queue manually. The option is overridden if a message delivery is forced with the -M, -qf, -Rf or -Sf options, and also while testing or verifying addresses using -bt or -bv. Otherwise, if a domain matches an item in `hold_domains`, no routing or delivery for that address is done, and it is deferred every time the message is looked at.

This option is intended as a temporary operational measure for delaying the delivery of mail while some problem is being sorted out, or some new configuration tested. If you just want to delay the processing of some domains until a queue run occurs, you should use `queue_domains` or `queue_smtp_domains`, not `hold_domains`.

A setting of `hold_domains` does not override Exim's code for removing messages from the queue if they have been there longer than the longest retry time in any retry rule. If you want to hold messages for longer than the normal retry times, insert a dummy retry rule with a long retry time.

### host_lookup

Type: host list, expanded
Default: unset

Exim does not look up the name of a calling host from its IP address unless it is required to compare against some host list, or the host matches `helo_try_verify_hosts` or `helo_verify_hosts`, or the host matches this option (which normally contains IP addresses rather than host names). The default configuration file contains

      host_lookup = *

which causes a lookup to happen for all hosts. If the expense of these lookups is felt to be too great, the setting can be changed or removed.

After a successful reverse lookup, Exim does a forward lookup on the name it has obtained, to verify that it yields the IP address that it started with. If this check fails, Exim behaves as if the name lookup failed.

After any kind of failure, the host name (in `$sender_host_name`) remains unset, and `$host_lookup_failed` is set to the string “1”. See also `dns_again_means_nonexist`, `helo_lookup_domains`, and `verify = reverse_host_lookup` in ACLs.

### host_lookup_order

Type: string list
Default: `bydns:byaddr`

This option specifies the order of different lookup methods when Exim is trying to find a host name from an IP address. The default is to do a DNS lookup first, and then to try a local lookup (using gethostbyaddr() or equivalent) if that fails. You can change the order of these lookups, or omit one entirely, if you want.

Warning: the “byaddr” method does not always yield aliases when there are multiple PTR records in the DNS and the IP address is not listed in /etc/hosts. Different operating systems give different results in this case. That is why the default tries a DNS lookup first.

### host_reject_connection

Type: host list, expanded
Default: unset

If this option is set, incoming SMTP calls from the hosts listed are rejected as soon as the connection is made. This option is obsolete, and retained only for backward compatibility, because nowadays the ACL specified by `acl_smtp_connect` can also reject incoming connections immediately.

The ability to give an immediate rejection (either by this option or using an ACL) is provided for use in unusual cases. Many hosts will just try again, sometimes without much delay. Normally, it is better to use an ACL to reject incoming messages at a later stage, such as after RCPT commands. See chapter 38.

### hosts_treat_as_local

Type: domain list, expanded
Default: unset

If this option is set, any host names that match the domain list are treated as if they were the local host when Exim is scanning host lists obtained from MX records or other sources. Note that the value of this option is a domain list, not a host list, because it is always used to check host names, not IP addresses.

This option also applies when Exim is matching the special items `@mx_any`, `@mx_primary`, and `@mx_secondary` in a domain list (see section 10.8), and when checking the `hosts` option in the smtp transport for the local host (see the `allow_localhost` option in that transport). See also `local_interfaces`, `extra_local_interfaces`, and chapter 13, which contains a discussion about local network interfaces and recognising the local host.

### ignore_bounce_errors_after

Type: time
Default: 10w

This option affects the processing of bounce messages that cannot be delivered, that is, those that suffer a permanent delivery failure. (Bounce messages that suffer temporary delivery failures are of course retried in the usual way.)

After a permanent delivery failure, bounce messages are frozen, because there is no sender to whom they can be returned. When a frozen bounce message has been on the queue for more than the given time, it is unfrozen at the next queue run, and a further delivery is attempted. If delivery fails again, the bounce message is discarded. This makes it possible to keep failed bounce messages around for a shorter time than the normal maximum retry time for frozen messages. For example,

      ignore_bounce_errors_after = 12h

retries failed bounce message deliveries after 12 hours, discarding any further failures. If the value of this option is set to a zero time period, bounce failures are discarded immediately. Setting a very long time (as in the default value) has the effect of disabling this option. For ways of automatically dealing with other kinds of frozen message, see `auto_thaw` and `timeout_frozen_after`.

### ignore_fromline_hosts

Type: host list, expanded
Default: unset

Some broken SMTP clients insist on sending a UUCP-like “From” line before the headers of a message. By default this is treated as the start of the message's body, which means that any following headers are not recognized as such. Exim can be made to ignore it by setting `ignore_fromline_hosts` to match those hosts that insist on sending it. If the sender is actually a local process rather than a remote host, and is using -bs to inject the messages, `ignore_fromline_local` must be set to achieve this effect.

### ignore_fromline_local

Type: boolean
Default: false

See `ignore_fromline_hosts` above.

### keep_malformed

Type: time
Default: 4d

This option specifies the length of time to keep messages whose spool files have been corrupted in some way. This should, of course, never happen. At the next attempt to deliver such a message, it gets removed. The incident is logged.

### ldap_default_servers

Type: string list
Default: unset

This option provides a list of LDAP servers which are tried in turn when an LDAP query does not contain a server. See section 9.11 for details of LDAP queries. This option is available only when Exim has been built with LDAP support.

### ldap_version

Type: integer
Default: unset

This option can be used to force Exim to set a specific protocol version for LDAP. If it option is unset, it is shown by the -bP command line option as -1. When this is the case, the default is 3 if LDAP_VERSION3 is defined in the LDAP headers; otherwise it is 2. This option is available only when Exim has been built with LDAP support.

### local_from_check

Type: boolean
Default: true

When a message is submitted locally (that is, not over a TCP/IP connection) by an untrusted user, Exim removes any existing Sender: header line, and checks that the From: header line matches the login of the calling user. You can use `local_from_prefix` and `local_from_suffix` to permit affixes on the local part. If the From: header line does not match, Exim adds a Sender: header with an address constructed from the calling user's login and the default qualify domain.

If `local_from_check` is set false, the From: header check is disabled, and no Sender: header is ever added. If, in addition, you want to retain Sender: header lines supplied by untrusted users, you must also set `local_sender_retain` to be true.

These options affect only the header lines in the message. The envelope sender is still forced to be the login id at the qualify domain unless `untrusted_set_sender` permits the user to supply an envelope sender. Section 44.14 has more details about Sender: processing.

### local_from_prefix

Type: string
Default: unset

When Exim checks the From: header line of locally submitted messages for matching the login id (see `local_from_check` above), it can be configured to ignore certain prefixes and suffixes in the local part of the address. This is done by setting `local_from_prefix` and/or `local_from_suffix` to appropriate lists, in the same form as the `local_part_prefix` and `local_part_suffix` router options (see chapter 15). For example, if

      local_from_prefix = *-

is set, a From: line containing

      From: anything-user@your.domain.example

will not cause a Sender: header to be added if user@your.domain.example matches the actual sender address that is constructed from the login name and qualify domain.

### local_from_suffix

Type: string
Default: unset

See `local_from_prefix` above.

### local_interfaces

Type: string list
Default: see below

This option controls which network interfaces are used by the daemon for listening; they are also used to identify the local host when routing. Chapter 13 contains a full description of this option and the related options `extra_local_interfaces` and `hosts_treat_as_local`. The default value for `local_interfaces` is

      local_interfaces = 0.0.0.0

when Exim is built without IPv6 support; otherwise it is

      local_interfaces = <; ::0 ; 0.0.0.0

### local_scan_timeout

Type: time
Default: 5m

This timeout applies to the local_scan() function (see chapter 39). Zero means “no timeout”. If the timeout is exceeded, the incoming message is rejected with a temporary error if it is an SMTP message. For a non-SMTP message, the message is dropped and Exim ends with a non-zero code. The incident is logged on the main and reject logs.

### local_sender_retain

Type: boolean
Default: false

When a message is submitted locally (that is, not over a TCP/IP connection) by an untrusted user, Exim removes any existing Sender: header line. If you do not want this to happen, you must set `local_sender_retain`, and you must also set `local_from_check` to be false (Exim will complain if you do not). Section 44.14 has more details about Sender: processing.

### localhost_number

Type: string, expanded
Default: unset

Exim's message ids are normally unique only within the local host. If uniqueness among a set of hosts is required, each host must set a different value for the `localhost_number` option. The string is expanded immediately after reading the configuration file (so that a number can be computed from the host name, for example) and the result of the expansion must be a number in the range 0–16 (or 0–10 on operating systems with case-insensitive file systems). This is available in subsequent string expansions via the variable `$localhost_number`. When `localhost_number is set`, the final two characters of the message id, instead of just being a fractional part of the time, are computed from the time and the local host number as described in section 3.4.

### log_file_path

Type: string list, expanded
Default: set at compile time

This option sets the path which is used to determine the names of Exim's log files, or indicates that logging is to be to syslog, or both. It is expanded when Exim is entered, so it can, for example, contain a reference to the host name. If no specific path is set for the log files at compile or run time, they are written in a sub-directory called log in Exim's spool directory. Chapter 45 contains further details about Exim's logging, and section 45.1 describes how the contents of `log_file_path` are used. If this string is fixed at your installation (contains no expansion variables) it is recommended that you do not set this option in the configuration file, but instead supply the path using LOG_FILE_PATH in Local/Makefile so that it is available to Exim for logging errors detected early on – in particular, failure to read the configuration file.

### log_selector

Type: string
Default: unset

This option can be used to reduce or increase the number of things that Exim writes to its log files. Its argument is made up of names preceded by plus or minus characters. For example:

      log_selector = +arguments -retry_defer

A list of possible names and what they control is given in the chapter on logging, in section 45.15.

### log_timezone

Type: boolean
Default: false

By default, the timestamps on log lines are in local time without the timezone. This means that if your timezone changes twice a year, the timestamps in log lines are ambiguous for an hour when the clocks go back. One way of avoiding this problem is to set the timezone to UTC. An alternative is to set `log_timezone` true. This turns on the addition of the timezone offset to timestamps in log lines. Turning on this option can add quite a lot to the size of log files because each line is extended by 6 characters. Note that the `$tod_log` variable contains the log timestamp without the zone, but there is another variable called `$tod_zone` that contains just the timezone offset.

### lookup_open_max

Type: integer
Default: 25

This option limits the number of simultaneously open files for single-key lookups that use regular files (that is, lsearch, dbm, and cdb). Exim normally keeps these files open during routing, because often the same file is required several times. If the limit is reached, Exim closes the least recently used file. Note that if you are using the ndbm library, it actually opens two files for each logical DBM database, though it still counts as one for the purposes of `lookup_open_max`. If you are getting “too many open files” errors with NDBM, you need to reduce the value of `lookup_open_max`.

### max_username_length

Type: integer
Default: 0

Some operating systems are broken in that they truncate long arguments to getpwnam() to eight characters, instead of returning “no such user”. If this option is set greater than zero, any attempt to call getpwnam() with an argument that is longer behaves as if getpwnam() failed.

### message_body_visible

Type: integer
Default: 500

This option specifies how much of a message's body is to be included in the `$message_body` and `$message_body_end` expansion variables.

### message_id_header_domain

Type: string, expanded
Default: unset

If this option is set, the string is expanded and used as the right hand side (domain) of the Message-ID: header that Exim creates if a locally-originated incoming message does not have one. “Locally-originated” means “not received over TCP/IP.” Otherwise, the primary host name is used. Only letters, digits, dot and hyphen are accepted; any other characters are replaced by hyphens. If the expansion is forced to fail, or if the result is an empty string, the option is ignored.

### message_id_header_text

Type: string, expanded
Default: unset

If this variable is set, the string is expanded and used to augment the text of the Message-id: header that Exim creates if a locally-originated incoming message does not have one. The text of this header is required by RFC 2822 to take the form of an address. By default, Exim uses its internal message id as the local part, and the primary host name as the domain. If this option is set, it is expanded, and provided the expansion is not forced to fail, and does not yield an empty string, the result is inserted into the header immediately before the @, separated from the internal message id by a dot. Any characters that are illegal in an address are automatically converted into hyphens. This means that variables such as `$tod_log` can be used, because the spaces and colons will become hyphens.

### message_logs

Type: boolean
Default: true

If this option is turned off, per-message log files are not created in the msglog spool sub-directory. This reduces the amount of disk I/O required by Exim, by reducing the number of files involved in handling a message from a minimum of four (header spool file, body spool file, delivery journal, and per-message log) to three. The other major I/O activity is Exim's main log, which is not affected by this option.

### message_size_limit

Type: string, expanded
Default: 50M

This option limits the maximum size of message that Exim will process. The value is expanded for each incoming connection so, for example, it can be made to depend on the IP address of the remote host for messages arriving via TCP/IP. Note: This limit cannot be made to depend on a message's sender or any other properties of an individual message, because it has to be advertised in the server's response to EHLO. String expansion failure causes a temporary error. A value of zero means no limit, but its use is not recommended. See also `bounce_return_size_limit`.

Incoming SMTP messages are failed with a 552 error if the limit is exceeded; locally-generated messages either get a stderr message or a delivery failure message to the sender, depending on the -oe setting. Rejection of an oversized message is logged in both the main and the reject logs. See also the generic transport option `message_size_limit`, which limits the size of message that an individual transport can process.

### move_frozen_messages

Type: boolean
Default: false

This option, which is available only if Exim has been built with the setting

      SUPPORT_MOVE_FROZEN_MESSAGES=yes

in Local/Makefile, causes frozen messages and their message logs to be moved from the input and msglog directories on the spool to Finput and Fmsglog, respectively. There is currently no support in Exim or the standard utilities for handling such moved messages, and they do not show up in lists generated by -bp or by the Exim monitor.

### mysql_servers

Type: string list
Default: unset

This option provides a list of MySQL servers and associated connection data, to be used in conjunction with mysql lookups (see section 9.17). The option is available only if Exim has been built with MySQL support.

### never_users

Type: string list
Default: unset

Local message deliveries are normally run in processes that are setuid to the recipient, and remote deliveries are normally run under Exim's own uid and gid. It is usually desirable to prevent any deliveries from running as root, as a safety precaution.

When Exim is built, an option called FIXED_NEVER_USERS can be set to a list of users that must not be used for local deliveries. This list is fixed in the binary and cannot be overridden by the configuration file. By default, it contains just the single user name “root”. The `never_users` runtime option can be used to add more users to the fixed list.

If a message is to be delivered as one of the users on the fixed list or the `never_users` list, an error occurs, and delivery is deferred. A common example is

`` `never_users = root:daemon:bin`

Including root is redundant if it is also on the fixed list, but it does no harm. This option overrides the `pipe_as_creator` option of the pipe transport driver.

### oracle_servers

Type: string list
Default: unset

This option provides a list of Oracle servers and associated connection data, to be used in conjunction with oracle lookups (see section 9.17). The option is available only if Exim has been built with Oracle support.

### percent_hack_domains

Type: domain list, expanded
Default: unset

The “percent hack” is the convention whereby a local part containing a percent sign is re-interpreted as a new email address, with the percent replaced by @. This is sometimes called “source routing”, though that term is also applied to RFC 2822 addresses that begin with an @ character. If this option is set, Exim implements the percent facility for those domains listed, but no others. This happens before an incoming SMTP address is tested against an ACL.

Warning: The “percent hack” has often been abused by people who are trying to get round relaying restrictions. For this reason, it is best avoided if at all possible. Unfortunately, a number of less security-conscious MTAs implement it unconditionally. If you are running Exim on a gateway host, and routing mail through to internal MTAs without processing the local parts, it is a good idea to reject recipient addresses with percent characters in their local parts. Exim's default configuration does this.

### perl_at_start

Type: boolean
Default: false

This option is available only when Exim is built with an embedded Perl interpreter. See chapter 12 for details of its use.

### perl_startup

Type: string
Default: unset

This option is available only when Exim is built with an embedded Perl interpreter. See chapter 12 for details of its use.

### pgsql_servers

Type: string list
Default: unset

This option provides a list of PostgreSQL servers and associated connection data, to be used in conjunction with pgsql lookups (see section 9.17). The option is available only if Exim has been built with PostgreSQL support.

### pid_file_path

Type: string, expanded
Default: set at compile time

This option sets the name of the file to which the Exim daemon writes its process id. The string is expanded, so it can contain, for example, references to the host name:

      pid_file_path = /var/log/$primary_hostname/exim.pid

If no path is set, the pid is written to the file exim-daemon.pid in Exim's spool directory. The value set by the option can be overridden by the -oP command line option. A pid file is not written if a “non-standard” daemon is run by means of the -oX option, unless a path is explicitly supplied by -oP.

### pipelining_advertise_hosts

Type: host list, expanded
Default: *

This option can be used to suppress the advertisement of the SMTP PIPELINING extension to specific hosts. When PIPELINING is not advertised and `smtp_enforce_sync` is true, an Exim server enforces strict synchronization for each SMTP command and response. When PIPELINING is advertised, Exim assumes that clients will use it; “out of order” commands that are “expected” do not count as protocol errors (see `smtp_max_synprot_errors`).

### preserve_message_logs

Type: boolean
Default: false

If this option is set, message log files are not deleted when messages are completed. Instead, they are moved to a sub-directory of the spool directory called msglog.OLD, where they remain available for statistical or debugging purposes. This is a dangerous option to set on systems with any appreciable volume of mail. Use with care!

### primary_hostname

Type: string
Default: see below

This specifies the name of the current host. It is used in the default EHLO or HELO command for outgoing SMTP messages (changeable via the `helo_data` option in the smtp transport), and as the default for `qualify_domain`. If it is not set, Exim calls uname() to find it. If this fails, Exim panics and dies. If the name returned by uname() contains only one component, Exim passes it to gethostbyname() (or getipnodebyname() when available) in order to obtain the fully qualified version.

The value of `$primary_hostname` is also used by default in some SMTP response messages from an Exim server. This can be changed dynamically by setting `smtp_active_hostname`.

### print_topbitchars

Type: boolean
Default: false

By default, Exim considers only those characters whose codes lie in the range 32–126 to be printing characters. In a number of circumstances (for example, when writing log entries) non-printing characters are converted into escape sequences, primarily to avoid messing up the layout. If `print_topbitchars` is set, code values of 128 and above are also considered to be printing characters.

### process_log_path

Type: string
Default: unset

This option sets the name of the file to which an Exim process writes its “process log” when sent a USR1 signal. This is used by the exiwhat utility script. If this option is unset, the file called exim-process.info in Exim's spool directory is used. The ability to specify the name explicitly can be useful in environments where two different Exims are running, using different spool directories.

### prod_requires_admin

Type: boolean
Default: true

The -M, -R, and -q command-line options require the caller to be an admin user unless `prod_requires_admin` is set false. See also `queue_list_requires_admin`.

### qualify_domain

Type: string
Default: see below

This option specifies the domain name that is added to any sender addresses that do not have a domain qualification. It also applies to recipient addresses if `qualify_recipient` is not set. Such addresses are accepted by default only for locally-generated messages. Messages from external sources must always contain fully qualified addresses, unless the sending host matches `sender_unqualified_hosts` or `recipient_unqualified_hosts` (as appropriate), in which case incoming addresses are qualified with `qualify_domain` or `qualify_recipient` as necessary. Internally, Exim always works with fully qualified addresses. If `qualify_domain` is not set, it defaults to the `primary_hostname` value.

### qualify_recipient

Type: string
Default: see below

This specifies the domain name that is added to any recipient addresses that do not have a domain qualification. Such addresses are accepted by default only for locally-generated messages. Messages from external sources must always contain fully qualified recipient addresses, unless the sending host matches `recipient_unqualified_hosts`, in which case incoming recipient addresses are qualified with `qualify_recipient`. If `qualify_recipient` is not set, it defaults to the `qualify_domain` value.

### queue_domains

Type: domain list, expanded
Default: unset

This option lists domains for which immediate delivery is not required. A delivery process is started whenever a message is received, but only those domains that do not match are processed. All other deliveries wait until the next queue run. See also `hold_domains` and `queue_smtp_domains`.

### queue_list_requires_admin

Type: boolean
Default: true

The -bp command-line option, which lists the messages that are on the queue, requires the caller to be an admin user unless `queue_list_requires_admin` is set false. See also `prod_requires_admin`.

### queue_only

Type: boolean
Default: false

If `queue_only` is set, a delivery process is not automatically started whenever a message is received. Instead, the message waits on the queue for the next queue run. Even if `queue_only` is false, incoming messages may not get delivered immediately when certain conditions (such as heavy load) occur.

The -odq command line has the same effect as `queue_only`. The -odb and -odi command line options override `queue_only` unless `queue_only_override` is set false. See also `queue_only_file`, `queue_only_load`, and `smtp_accept_queue`.

### queue_only_file

Type: string
Default: unset

This option can be set to a colon-separated list of absolute path names, each one optionally preceded by “smtp”. When Exim is receiving a message, it tests for the existence of each listed path using a call to stat(). For each path that exists, the corresponding queuing option is set. For paths with no prefix, `queue_only` is set; for paths prefixed by “smtp”, `queue_smtp_domains` is set to match all domains. So, for example,

      queue_only_file = smtp/some/file

causes Exim to behave as if `queue_smtp_domains` were set to “*” whenever /some/file exists.

### queue_only_load

Type: fixed-point
Default: unset

If the system load average is higher than this value, incoming messages from all sources are queued, and no automatic deliveries are started. If this happens during local or remote SMTP input, all subsequent messages on the same connection are queued. Deliveries will subsequently be performed by queue runner processes. This option has no effect on ancient operating systems on which Exim cannot determine the load average. See also `deliver_queue_load_max` and `smtp_load_reserve`.

### queue_only_override

Type: boolean
Default: true

When this option is true, the `-odx-` command line options override the setting of `queue_only` or `queue_only_file` in the configuration file. If `queue_only_override` is set false, the `-odx-` options cannot be used to override; they are accepted, but ignored.

### queue_run_in_order

Type: boolean
Default: false

If this option is set, queue runs happen in order of message arrival instead of in an arbitrary order. For this to happen, a complete list of the entire queue must be set up before the deliveries start. When the queue is all in a single directory (the default), this happens anyway, but if `split_spool_directory` is set it does not – for delivery in random order, the sub-directories are processed one at a time (in random order), to avoid setting up one huge list. Thus, setting `queue_run_in_order` with `split_spool_directory` may degrade performance when the queue is large. In most situations, `queue_run_in_order` should not be set.

### queue_run_max

Type: integer
Default: 5

This controls the maximum number of queue runner processes that an Exim daemon can run simultaneously. This does not mean that it starts them all at once, but rather that if the maximum number are still running when the time comes to start another one, it refrains from starting another one. This can happen with very large queues and/or very sluggish deliveries. This option does not, however, interlock with other processes, so additional queue runners can be started by other means, or by killing and restarting the daemon.

### queue_smtp_domains

Type: domain list, expanded
Default: unset

When this option is set, a delivery process is started whenever a message is received, routing is performed, and local deliveries take place. However, if any SMTP deliveries are required for domains that match `queue_smtp_domains`, they are not immediately delivered, but instead the message waits on the queue for the next queue run. Since routing of the message has taken place, Exim knows to which remote hosts it must be delivered, and so when the queue run happens, multiple messages for the same host are delivered over a single SMTP connection. The -odqs command line option causes all SMTP deliveries to be queued in this way, and is equivalent to setting `queue_smtp_domains` to “*”. See also `hold_domains` and `queue_domains`.

### receive_timeout

Type: time
Default: 0s

This option sets the timeout for accepting a non-SMTP message, that is, the maximum time that Exim waits when reading a message on the standard input. If the value is zero, it will wait for ever. This setting is overridden by the -or command line option. The timeout for incoming SMTP messages is controlled by `smtp_receive_timeout`.

### received_header_text

Type: string, expanded
Default: see below

This string defines the contents of the Received: message header that is added to each message, except for the timestamp, which is automatically added on at the end (preceded by a semicolon). The string is expanded each time it is used. If the expansion yields an empty string, no Received: header line is added to the message. Otherwise, the string should start with the text “Received:” and conform to the RFC 2822 specification for Received: header lines. The default setting is:

      received_header_text = Received: \
          ${if def:sender_rcvhost {from $sender_rcvhost\n\t}\
          {${if def:sender_ident {from $sender_ident }}\
          ${if def:sender_helo_name {(helo=$sender_helo_name)\n\t}}}}\
          by $primary_hostname \
          ${if def:received_protocol {with $received_protocol}} \
          ${if def:tls_cipher {($tls_cipher)\n\t}}\
          (Exim $version_number)\n\t\
          id $message_id\
          ${if def:received_for {\n\tfor $received_for}}

Note the use of quotes, to allow the sequences `\n` and `\t` to be used for newlines and tabs, respectively. The reference to the TLS cipher is omitted when Exim is built without TLS support. The use of conditional expansions ensures that this works for both locally generated messages and messages received from remote hosts, giving header lines such as the following:

      Received: from scrooge.carol.example ([192.168.12.25] ident=root)
              by marley.carol.example with esmtp (Exim 4.00)
              id 16IOWa-00019l-00
              for chas@dickens.example; Tue, 25 Dec 2001 14:43:44 +0000
      Received: by scrooge.carol.example with local (Exim 4.00)
              id 16IOWW-000083-00; Tue, 25 Dec 2001 14:43:41 +0000

Until the body of the message has been received, the timestamp is the time when the message started to be received. Once the body has arrived, and all policy checks have taken place, the timestamp is updated to the time at which the message was accepted.

### received_headers_max

Type: integer
Default: 30

When a message is to be delivered, the number of Received: headers is counted, and if it is greater than this parameter, a mail loop is assumed to have occurred, the delivery is abandoned, and an error message is generated. This applies to both local and remote deliveries.

### recipient_unqualified_hosts

Type: host list, expanded
Default: unset

This option lists those hosts from which Exim is prepared to accept unqualified recipient addresses in message envelopes. The addresses are made fully qualified by the addition of the `qualify_recipient` value. This option also affects message header lines. Exim does not reject unqualified recipient addresses in headers, but it qualifies them only if the message came from a host that matches `recipient_unqualified_hosts`, or if the message was submitted locally (not using TCP/IP), and the -bnq option was not set.

### recipients_max

Type: integer
Default: 0

If this option is set greater than zero, it specifies the maximum number of original recipients for any message. Additional recipients that are generated by aliasing or forwarding do not count. SMTP messages get a 452 response for all recipients over the limit; earlier recipients are delivered as normal. Non-SMTP messages with too many recipients are failed, and no deliveries are done. Note that the RFCs specify that an SMTP server should accept at least 100 RCPT commands in a single message.

### recipients_max_reject

Type: boolean
Default: false

If this option is set true, Exim rejects SMTP messages containing too many recipients by giving 552 errors to the surplus RCPT commands, and a 554 error to the eventual DATA command. Otherwise (the default) it gives a 452 error to the surplus RCPT commands and accepts the message on behalf of the initial set of recipients. The remote server should then re-send the message for the remaining recipients at a later time.

### remote_max_parallel

Type: integer
Default: 2

This option controls parallel delivery of one message to a number of remote hosts. If the value is less than 2, parallel delivery is disabled, and Exim does all the remote deliveries for a message one by one. Otherwise, if a single message has to be delivered to more than one remote host, or if several copies have to be sent to the same remote host, up to `remote_max_parallel` deliveries are done simultaneously. If more than `remote_max_parallel` deliveries are required, the maximum number of processes are started, and as each one finishes, another is begun. The order of starting processes is the same as if sequential delivery were being done, and can be controlled by the `remote_sort_domains` option. If parallel delivery takes place while running with debugging turned on, the debugging output from each delivery process is tagged with its process id.

This option controls only the maximum number of parallel deliveries for one message in one Exim delivery process. Because Exim has no central queue manager, there is no way of controlling the total number of simultaneous deliveries if the configuration allows a delivery attempt as soon as a message is received. If you want to control the total number of deliveries on the system, you need to set the `queue_only` option. This ensures that all incoming messages are added to the queue without starting a delivery process. Then set up an Exim daemon to start queue runner processes at appropriate intervals (probably fairly often, for example, every minute), and limit the total number of queue runners by setting the `queue_run_max` parameter. Because each queue runner delivers only one message at a time, the maximum number of deliveries that can then take place at once is `queue_run_max` multiplied by `remote_max_parallel`.

If it is purely remote deliveries you want to control, use `queue_smtp` instead of `queue_only`. This has the added benefit of doing the SMTP routing before queuing, so that several messages for the same host will eventually get delivered down the same connection.

### remote_sort_domains

Type: domain list, expanded
Default: unset

When there are a number of remote deliveries for a message, they are sorted by domain into the order given by this list. For example,

      remote_sort_domains = *.cam.ac.uk:*.uk

would attempt to deliver to all addresses in the cam.ac.uk domain first, then to those in the `uk` domain, then to any others.

### retry_data_expire

Type: time
Default: 7d

This option sets a “use before” time on retry information in Exim's hints database. Any older retry data is ignored. This means that, for example, once a host has not been tried for 7 days, Exim behaves as if it has no knowledge of past failures.

### retry_interval_max

Type: time
Default: 24h

Chapter 32 describes Exim's mechanisms for controlling the intervals between delivery attempts for messages that cannot be delivered straight away. This option sets an overall limit to the length of time between retries.

### return_path_remove

Type: boolean
Default: true

RFC 2821, section 4.4, states that an SMTP server must insert a Return-path: header line into a message when it makes a “final delivery”. The Return-path: header preserves the sender address as received in the MAIL command. This description implies that this header should not be present in an incoming message. If `return_path_remove` is true, any existing Return-path: headers are removed from messages at the time they are received. Exim's transports have options for adding Return-path: headers at the time of delivery. They are normally used only for final local deliveries.

### return_size_limit

Type: integer
Default: 100K

This option is an obsolete synonym for `bounce_return_size_limit`.

### rfc1413_hosts

Type: host list, expanded
Default: *

RFC 1413 identification calls are made to any client host which matches an item in the list.

### rfc1413_query_timeout

Type: time
Default: 30s

This sets the timeout on RFC 1413 identification calls. If it is set to zero, no RFC 1413 calls are ever made.

### sender_unqualified_hosts

Type: host list, expanded
Default: unset

This option lists those hosts from which Exim is prepared to accept unqualified sender addresses. The addresses are made fully qualified by the addition of `qualify_domain`. This option also affects message header lines. Exim does not reject unqualified addresses in headers that contain sender addresses, but it qualifies them only if the message came from a host that matches `sender_unqualified_hosts`, or if the message was submitted locally (not using TCP/IP), and the -bnq option was not set.

### smtp_accept_keepalive

Type: boolean
Default: true

This option controls the setting of the SO_KEEPALIVE option on incoming TCP/IP socket connections. When set, it causes the kernel to probe idle connections periodically, by sending packets with “old” sequence numbers. The other end of the connection should send an acknowledgement if the connection is still okay or a reset if the connection has been aborted. The reason for doing this is that it has the beneficial effect of freeing up certain types of connection that can get stuck when the remote host is disconnected without tidying up the TCP/IP call properly. The keepalive mechanism takes several hours to detect unreachable hosts.

### smtp_accept_max

Type: integer
Default: 20

This option specifies the maximum number of simultaneous incoming SMTP calls that Exim will accept. It applies only to the listening daemon; there is no control (in Exim) when incoming SMTP is being handled by inetd. If the value is set to zero, no limit is applied. However, it is required to be non-zero if either `smtp_accept_max_per_host` or `smtp_accept_queue` is set. See also `smtp_accept_reserve`.

### smtp_accept_max_nonmail

Type: integer
Default: 10

Exim counts the number of “non-mail” commands in an SMTP session, and drops the connection if there are too many. This option defines “too many”. The check catches some denial-of-service attacks, repeated failing AUTHs, or a mad client looping sending EHLO, for example. The check is applied only if the client host matches `smtp_accept_max_nonmail_hosts`.

When a new message is expected, one occurrence of RSET is not counted. This allows a client to send one RSET between messages (this is not necessary, but some clients do it). Exim also allows one uncounted occurence of HELO or EHLO, and one occurrence of STARTTLS between messages. After starting up a TLS session, another EHLO is expected, and so it too is not counted. The first occurrence of AUTH in a connection, or immediately following STARTTLS is not counted. Otherwise, all commands other than MAIL, RCPT, DATA, and QUIT are counted.

### smtp_accept_max_nonmail_hosts

Type: host list, expanded
Default: *

You can control which hosts are subject to the `smtp_accept_max_nonmail` check by setting this option. The default value makes it apply to all hosts. By changing the value, you can exclude any badly-behaved hosts that you have to live with.

### smtp_accept_max_per_connection

Type: integer
Default: 1000

The value of this option limits the number of MAIL commands that Exim is prepared to accept over a single SMTP connection, whether or not each command results in the transfer of a message. After the limit is reached, a 421 response is given to subsequent MAIL commands. This limit is a safety precaution against a client that goes mad (incidents of this type have been seen).

### smtp_accept_max_per_host

Type: string, expanded
Default: unset

This option restricts the number of simultaneous IP connections from a single host (strictly, from a single IP address) to the Exim daemon. The option is expanded, to enable different limits to be applied to different hosts by reference to `$sender_host_address`. Once the limit is reached, additional connection attempts from the same host are rejected with error code 421. The default value of zero imposes no limit. If this option is set, it is required that `smtp_accept_max` be non-zero.

Warning: When setting this option you should not use any expansion constructions that take an appreciable amount of time. The expansion and test happen in the main daemon loop, in order to reject additional connections without forking additional processes (otherwise a denial-of-service attack could cause a vast number or processes to be created). While the daemon is doing this processing, it cannot accept any other incoming connections.

### smtp_accept_queue

Type: integer
Default: 0

If the number of simultaneous incoming SMTP calls handled via the listening daemon exceeds this value, messages received by SMTP are just placed on the queue; no delivery processes are started automatically. A value of zero implies no limit, and clearly any non-zero value is useful only if it is less than the `smtp_accept_max` value (unless that is zero). See also `queue_only`, `queue_only_load`, `queue_smtp_domains`, and the various -od command line options.

### smtp_accept_queue_per_connection

Type: integer
Default: 10

This option limits the number of delivery processes that Exim starts automatically when receiving messages via SMTP, whether via the daemon or by the use of -bs or -bS. If the value of the option is greater than zero, and the number of messages received in a single SMTP session exceeds this number, subsequent messages are placed on the queue, but no delivery processes are started. This helps to limit the number of Exim processes when a server restarts after downtime and there is a lot of mail waiting for it on other systems. On large systems, the default should probably be increased, and on dial-in client systems it should probably be set to zero (that is, disabled).

### smtp_accept_reserve

Type: integer
Default: 0

When `smtp_accept_max` is set greater than zero, this option specifies a number of SMTP connections that are reserved for connections from the hosts that are specified in `smtp_reserve_hosts`. The value set in `smtp_accept_max` includes this reserve pool. The specified hosts are not restricted to this number of connections; the option specifies a minimum number of connection slots for them, not a maximum. It is a guarantee that that group of hosts can always get at least `smtp_accept_reserve` connections.

For example, if `smtp_accept_max` is set to 50 and `smtp_accept_reserve` is set to 5, once there are 45 active connections (from any hosts), new connections are accepted only from hosts listed in `smtp_reserve_hosts`. See also `smtp_accept_max_per_host`.

### smtp_active_hostname

Type: string, expanded
Default: unset

This option is provided for multi-homed servers that want to masquerade as several different hosts. At the start of an SMTP connection, its value is expanded and used instead of the value of `$primary_hostname` in SMTP responses. For example, it is used as domain name in the response to an incoming HELO or EHLO command. If this option is unset, or if its expansion is forced to fail, or if the expansion results in an empty string, the value of `$primary_hostname` is used. Other expansion failures cause a message to be written to the main and panic logs, and the SMTP command receives a temporary error. Typically, the value of `smtp_active_hostname` depends on the incoming interface address. For example:

      smtp_active_hostname = ${if eq{$interface_address}{10.0.0.1}\
        {cox.mydomain}{box.mydomain}}

If you set `smtp_active_hostname`, you probably also want to set `smtp_banner`, since its default value references `$primary_hostname`.

### smtp_banner

Type: string, expanded
Default: see below

This string, which is expanded every time it is used, is output as the initial positive response to an SMTP connection. The default setting is:

      smtp_banner = $primary_hostname ESMTP Exim $version_number \
        $tod_full

Failure to expand the string causes a panic error. If you want to create a multiline response to the initial SMTP connection, use “\n” in the string at appropriate points, but not at the end. Note that the 220 code is not included in this string. Exim adds it automatically (several times in the case of a multiline response).

### smtp_check_spool_space

Type: boolean
Default: true

When this option is set, if an incoming SMTP session encounters the SIZE option on a MAIL command, it checks that there is enough space in the spool directory's partition to accept a message of that size, while still leaving free the amount specified by `check_spool_space` (even if that value is zero). If there isn't enough space, a temporary error code is returned.

### smtp_connect_backlog

Type: integer
Default: 20

This option specifies a maximum number of waiting SMTP connections. Exim passes this value to the TCP/IP system when it sets up its listener. Once this number of connections are waiting for the daemon's attention, subsequent connection attempts are refused at the TCP/IP level. At least, that is what the manuals say; in some circumstances such connection attempts have been observed to time out instead. For large systems it is probably a good idea to increase the value (to 50, say). It also gives some protection against denial-of-service attacks by SYN flooding.

### smtp_enforce_sync

Type: boolean
Default: true

The SMTP protocol specification requires the client to wait for a response from the server at certain points in the dialogue. Without PIPELINING these synchronization points are after every command; with PIPELINING they are fewer, but they still exist. Some spamming sites send out a complete set of SMTP commands without waiting for any response. Exim protects against this by rejecting a message if the client has sent further input when it should not have. The error response “554 SMTP synchronization error” is sent, and the connection is dropped. Testing for this error cannot be perfect because of transmission delays (unexpected input may be on its way but not yet received when Exim checks). However, it does detect many instances. The check can be disabled by setting `smtp_enforce_sync` false. See also `pipelining_advertise_hosts`.

### smtp_etrn_command

Type: string, expanded
Default: unset

If this option is set, the given command is run whenever an SMTP ETRN command is received from a host that is permitted to issue such commands (see chapter 38). The string is split up into separate arguments which are independently expanded. The expansion variable `$domain` is set to the argument of the ETRN command, and no syntax checking is done on it. For example:

      smtp_etrn_command = /etc/etrn_command $domain $sender_host_address

A new process is created to run the command, but Exim does not wait for it to complete. Consequently, its status cannot be checked. If the command cannot be run, a line is written to the panic log, but the ETRN caller still receives a 250 success response. Exim is normally running under its own uid when receiving SMTP, so it is not possible for it to change the uid before running the command.

### smtp_etrn_serialize

Type: boolean
Default: true

When this option is set, it prevents the simultaneous execution of more than one identical command as a result of ETRN in an SMTP connection. See section 43.9 for details.

### smtp_load_reserve

Type: fixed-point
Default: unset

If the system load average ever gets higher than this, incoming SMTP calls are accepted only from those hosts that match an entry in `smtp_reserve_hosts`. If `smtp_reserve_hosts` is not set, no incoming SMTP calls are accepted when the load is over the limit. The option has no effect on ancient operating systems on which Exim cannot determine the load average. See also `deliver_queue_load_max` and `queue_only_load`.

### smtp_max_synprot_errors

Type: integer
Default: 3

Exim rejects SMTP commands that contain syntax or protocol errors. In particular, a syntactically invalid email address, as in this command:

      RCPT TO:<abc xyz@a.b.c>

causes immediate rejection of the command, before any other tests are done. (The ACL cannot be run if there is no valid address to set up for it.) An example of a protocol error is receiving RCPT before MAIL. If there are too many syntax or protocol errors in one SMTP session, the connection is dropped. The limit is set by this option.

When the PIPELINING extension to SMTP is in use, some protocol errors are “expected”, for instance, a RCPT command after a rejected MAIL command. Exim assumes that PIPELINING will be used if it advertises it (see `pipelining_advertise_hosts`), and in this situation, “expected” errors do not count towards the limit.

### smtp_max_unknown_commands

Type: integer
Default: 3

If there are too many unrecognized commands in an incoming SMTP session, an Exim server drops the connection. This is a defence against some kinds of abuse that subvert web clients into making connections to SMTP ports; in these circumstances, a number of non-SMTP command lines are sent first.

### smtp_ratelimit_hosts

Type: host list, expanded
Default: unset

Some sites find it helpful to be able to limit the rate at which certain hosts can send them messages, and the rate at which an individual message can specify recipients. When a host matches `smtp_ratelimit_hosts`, the values of `smtp_ratelimit_mail` and `smtp_ratelimit_rcpt` are used to control the rate of acceptance of MAIL and RCPT commands in a single SMTP session, respectively. Each option, if set, must contain a set of four comma-separated values:

- A threshold, before which there is no rate limiting.

- An initial time delay. Unlike other times in Exim, numbers with decimal fractional parts are allowed here.

- A factor by which to increase the delay each time.

- A maximum value for the delay. This should normally be less than 5 minutes, because after that time, the client is liable to timeout the SMTP command.

For example, these settings have been used successfully at the site which first suggested this feature, for controlling mail from their customers:

      smtp_ratelimit_mail = 2,0.5s,1.05,4m
      smtp_ratelimit_rcpt = 4,0.25s,1.015,4m

The first setting specifies delays that are applied to MAIL commands after two have been received over a single connection. The initial delay is 0.5 seconds, increasing by a factor of 1.05 each time. The second setting applies delays to RCPT commands when more than four occur in a single message.

It is also possible to configure delays explicitly in ACLs. See section 38.11 for details.

### smtp_ratelimit_mail

Type: string
Default: unset

See `smtp_ratelimit_hosts` above.

### smtp_ratelimit_rcpt

Type: string
Default: unset

See `smtp_ratelimit_hosts` above.

### smtp_receive_timeout

Type: time
Default: 5m

This sets a timeout value for SMTP reception. It applies to all forms of SMTP input, including batch SMTP. If a line of input (either an SMTP command or a data line) is not received within this time, the SMTP connection is dropped and the message is abandoned. A line is written to the log containing one of the following messages:

      SMTP command timeout on connection from...
      SMTP data timeout on connection from...

The former means that Exim was expecting to read an SMTP command; the latter means that it was in the DATA phase, reading the contents of a message.

The value set by this option can be overridden by the -os command-line option. A setting of zero time disables the timeout, but this should never be used for SMTP over TCP/IP. (It can be useful in some cases of local input using -bs or -bS.) For non-SMTP input, the reception timeout is controlled by `receive_timeout` and -or.

### smtp_reserve_hosts

Type: host list, expanded
Default: unset

This option defines hosts for which SMTP connections are reserved; see `smtp_accept_reserve` and `smtp_load_reserve` above.

### smtp_return_error_details

Type: boolean
Default: false

In the default state, Exim uses bland messages such as “Administrative prohibition” when it rejects SMTP commands for policy reasons. Many sysadmins like this because it gives away little information to spammers. However, some other syadmins who are applying strict checking policies want to give out much fuller information about failures. Setting `smtp_return_error_details` true causes Exim to be more forthcoming. For example, instead of “Administrative prohibition”, it might give:

      550-Rejected after DATA: '>' missing at end of address:
      550 failing address in "From" header is: <user@dom.ain

### split_spool_directory

Type: boolean
Default: false

If this option is set, it causes Exim to split its input directory into 62 subdirectories, each with a single alphanumeric character as its name. The sixth character of the message id is used to allocate messages to subdirectories; this is the least significant base-62 digit of the time of arrival of the message.

Splitting up the spool in this way may provide better performance on systems where there are long mail queues, by reducing the number of files in any one directory. The msglog directory is also split up in a similar way to the input directory; however, if `preserve_message_logs` is set, all old msglog files are still placed in the single directory msglog.OLD.

It is not necessary to take any special action for existing messages when changing `split_spool_directory`. Exim notices messages that are in the “wrong” place, and continues to process them. If the option is turned off after a period of being on, the subdirectories will eventually empty and be automatically deleted.

When `split_spool_directory` is set, the behaviour of queue runner processes changes. Instead of creating a list of all messages in the queue, and then trying to deliver each one in turn, it constructs a list of those in one sub-directory and tries to deliver them, before moving on to the next sub-directory. The sub-directories are processed in a random order. This spreads out the scanning of the input directories, and uses less memory. It is particularly beneficial when there are lots of messages on the queue. However, if `queue_run_in_order` is set, none of this new processing happens. The entire queue has to be scanned and sorted before any deliveries can start.

### spool_directory

Type: string, expanded
Default: set at compile time

This defines the directory in which Exim keeps its spool, that is, the messages it is waiting to deliver. The default value is taken from the compile-time configuration setting, if there is one. If not, this option must be set. The string is expanded, so it can contain, for example, a reference to `$primary_hostname`.

If the spool directory name is fixed on your installation, it is recommended that you set it at build time rather than from this option, particularly if the log files are being written to the spool directory (see `log_file_path`). Otherwise log files cannot be used for errors that are detected early on, such as failures in the configuration file.

By using this option to override the compiled-in path, it is possible to run tests of Exim without using the standard spool.

### strip_excess_angle_brackets

Type: boolean
Default: false

If this option is set, redundant pairs of angle brackets round “route-addr” items in addresses are stripped. For example, <<xxx@a.b.c.d>> is treated as <xxx@a.b.c.d>. If this is in the envelope and the message is passed on to another MTA, the excess angle brackets are not passed on. If this option is not set, multiple pairs of angle brackets cause a syntax error.

### strip_trailing_dot

Type: boolean
Default: false

If this option is set, a trailing dot at the end of a domain in an address is ignored. If this is in the envelope and the message is passed on to another MTA, the dot is not passed on. If this option is not set, a dot at the end of a domain causes a syntax error. However, addresses in header lines are checked only when an ACL requests header syntax checking.

### syslog_duplication

Type: boolean
Default: true

When Exim is logging to syslog, it writes the log lines for its three separate logs at different syslog priorities so that they can in principle be separated on the logging hosts. Some installations do not require this separation, and in those cases, the duplication of certain log lines is a nuisance. If `syslog_duplication` is set false, only one copy of any particular log line is written to syslog. For lines that normally go to both the main log and the reject log, the reject log version (possibly containing message header lines) is written, at LOG_NOTICE priority. Lines that normally go to both the main and the panic log are written at the LOG_ALERT priority.

### syslog_facility

Type: string
Default: unset

This option sets the syslog “facility” name, used when Exim is logging to syslog. The value must be one of the strings “mail”, “user”, “news”, “uucp”, “daemon”, or “localx” where x is a digit between 0 and 7. If this option is unset, “mail” is used. See chapter 45 for details of Exim's logging.

### syslog_processname

Type: string
Default: `exim`

This option sets the syslog “ident” name, used when Exim is logging to syslog. The value must be no longer than 32 characters. See chapter 45 for details of Exim's logging.

### syslog_timestamp

Type: boolean
Default: true

If `syslog_timestamp` is set false, the timestamps on Exim's log lines are omitted when these lines are sent to syslog. See chapter 45 for details of Exim's logging.

### system_filter

Type: string, expanded
Default: unset

This option specifies an Exim filter file that is applied to all messages at the start of each delivery attempt, before any routing is done. System filters must be Exim filters; they cannot be Sieve filters. If the system filter generates any deliveries to files or pipes, or any new mail messages, the appropriate `system_filter_..._transport` option(s) must be set, to define which transports are to be used. Details of this facility are given in chapter 40.

### system_filter_directory_transport

Type: string, expanded
Default: unset

This sets the name of the transport driver that is to be used when the `save` command in a system message filter specifies a path ending in “/”, implying delivery of each message into a separate file in some directory. During the delivery, the variable `$address_file` contains the path name.

### system_filter_file_transport

Type: string, expanded
Default: unset

This sets the name of the transport driver that is to be used when the `save` command in a system message filter specifies a path not ending in “/”. During the delivery, the variable `$address_file` contains the path name.

### system_filter_group

Type: string
Default: unset

This option is used only when `system_filter_user` is also set. It sets the gid under which the system filter is run, overriding any gid that is associated with the user. The value may be numerical or symbolic.

### system_filter_pipe_transport

Type: string, expanded
Default: unset 7

This specifies the transport driver that is to be used when a `pipe` command is used in a system filter. During the delivery, the variable `$address_pipe` contains the pipe command.

### system_filter_reply_transport

Type: string, expanded
Default: unset

This specifies the transport driver that is to be used when a `mail` command is used in a system filter.

### system_filter_user

Type: string
Default: unset

If this option is not set, the system filter is run in the main Exim delivery process, as root. When the option is set, the system filter runs in a separate process, as the given user. Unless the string consists entirely of digits, it is looked up in the password data. Failure to find the named user causes a configuration error. The gid is either taken from the password data, or specified by `system_filter_group`. When the uid is specified numerically, `system_filter_group` is required to be set.

If the system filter generates any pipe, file, or reply deliveries, the uid under which the filter is run is used when transporting them, unless a transport option overrides. Normally you should set `system_filter_user` if your system filter generates these kinds of delivery.

### tcp_nodelay

Type: boolean
Default: true

If this option is set false, it stops the Exim daemon setting the TCP_NODELAY option on its listening sockets. Setting TCP_NODELAY turns off the “Nagle algorithm”, which is a way of improving network performance in interactive (character-by-character) situations. Turning it off should improve Exim's performance a bit, so that is what happens by default. However, it appears that some broken clients cannot cope, and time out. Hence this option. It affects only those sockets that are set up for listening by the daemon. Sockets created by the smtp transport for delivering mail always set TCP_NODELAY.

### timeout_frozen_after

Type: time
Default: 0s

If `timeout_frozen_after` is set to a time greater than zero, a frozen message of any kind that has been on the queue for longer than the given time is automatically cancelled at the next queue run. If it is a bounce message, it is just discarded; otherwise, a bounce is sent to the sender, in a similar manner to cancellation by the -Mg command line option. If you want to timeout frozen bounce messages earlier than other kinds of frozen message, see `ignore_bounce_errors_after`.

### timezone

Type: string
Default: unset

The value of `timezone` is used to set the environment variable TZ while running Exim (if it is different on entry). This ensures that all timestamps created by Exim are in the required timezone. If you want all your timestamps to be in UTC (aka GMT) you should set

      timezone = UTC

The default value is taken from TIMEZONE_DEFAULT in Local/Makefile, or, if that is not set, from the value of the TZ environment variable when Exim is built. If `timezone` is set to the empty string, either at build or run time, any existing TZ variable is removed from the environment when Exim runs. This is appropriate behaviour for obtaining wall-clock time on some, but unfortunately not all, operating systems.

### tls_advertise_hosts

Type: host list, expanded
Default: unset

When Exim is built with support for TLS encrypted connections, the availability of the STARTTLS command to set up an encrypted session is advertised in response to EHLO only to those client hosts that match this option. See chapter 37 for details of Exim's support for TLS.

### tls_certificate

Type: string, expanded
Default: unset

The value of this option is expanded, and must then be the absolute path to a file which contains the server's certificates. The server's private key is also assumed to be in this file if `tls_privatekey` is unset. See chapter 37 for further details.

Note: The certificates defined by this option are used only when Exim is receiving incoming messages as a server. If you want to supply certificates for use when sending messages as a client, you must set the `tls_certificate` option in the relevant smtp transport.

### tls_crl

Type: string, expanded
Default: unset

This option specifies a certificate revocation list. The expanded value must be the name of a file that contains a CRL in PEM format.

### tls_dhparam

Type: string, expanded
Default: unset

The value of this option is expanded, and must then be the absolute path to a file which contains the server's DH parameter values. This is used only for OpenSSL. When Exim is linked with GnuTLS, this option is ignored. See section 37.1 for further details.

### tls_privatekey

Type: string, expanded
Default: unset

The value of this option is expanded, and must then be the absolute path to a file which contains the server's private key. If this option is unset, the private key is assumed to be in the same file as the server's certificates. See chapter 37 for further details.

### tls_remember_esmtp

Type: boolean
Default: false

If this option is set true, Exim violates the RFCs by remembering that it is in “esmtp” state after successfully negotiating a TLS session. This provides support for broken clients that fail to send a new EHLO after starting a TLS session.

### tls_require_ciphers

Type: string, expanded
Default: unset

This option controls which ciphers can be used for incoming TLS connections. (The smtp transport has an option of the same name for controlling outgoing connections.) This option is expanded for each connection, so can be varied for different clients if required. The value of this option must be a list of permitted cipher suites. The OpenSSL and GnuTLS libraries handle cipher control in somewhat different ways. Details are given in section 37.2.

### tls_try_verify_hosts

Type: host list, expanded
Default: unset

See `tls_verify_hosts` below.

### tls_verify_certificates

Type: string, expanded
Default: unset

The value of this option is expanded, and must then be the absolute path to a file containing permitted certificates for clients that match `tls_verify_hosts` or `tls_try_verify_hosts`. Alternatively, if you are using OpenSSL, you can set `tls_verify_certificates` to the name of a directory containing certificate files. This does not work with GnuTLS; the option must be set to the name of a single file if you are using GnuTLS.

### tls_verify_hosts

Type: host list, expanded
Default: unset

This option, along with `tls_try_verify_hosts`, controls the checking of certificates from clients. The expected certificates are defined by `tls_verify_certificates`, which must be set. A configuration error occurs if either `tls_verify_hosts` or `tls_try_verify_hosts` is set and `tls_verify_certificates` is not set.

Any client that matches `tls_verify_hosts` is constrained by `tls_verify_certificates`. The client must present one of the listed certificates. If it does not, the connection is aborted.

A weaker form of checking is provided by `tls_try_verify_hosts`. If a client matches this option (but not `tls_verify_hosts`), Exim requests a certificate and checks it against `tls_verify_certificates`, but does not abort the connection if there is no certificate or if it does not match. This state can be detected in an ACL, which makes it possible to implement policies such as “accept for relay only if a verified certificate has been received, but accept for local delivery if encrypted, even without a verified certificate”.

Client hosts that match neither of these lists are not asked to present certificates.

### trusted_groups

Type: string list
Default: unset

If this option is set, any process that is running in one of the listed groups, or which has one of them as a supplementary group, is trusted. The groups can be specified numerically or by name. See section 5.2 for details of what trusted callers are permitted to do. If neither `trusted_groups` nor `trusted_users` is set, only root and the Exim user are trusted.

### trusted_users

Type: string list
Default: unset

If this option is set, any process that is running as one of the listed users is trusted. The users can be specified numerically or by name. See section 5.2 for details of what trusted callers are permitted to do. If neither `trusted_groups` nor `trusted_users` is set, only root and the Exim user are trusted.

### unknown_login

Type: string, expanded
Default: unset

This is a specialized feature for use in unusual configurations. By default, if the uid of the caller of Exim cannot be looked up using getpwuid(), Exim gives up. The `unknown_login` option can be used to set a login name to be used in this circumstance. It is expanded, so values like `user$caller_uid` can be set. When `unknown_login` is used, the value of `unknown_username` is used for the user's real name (gecos field), unless this has been set by the -F option.

### unknown_username

Type: string
Default: unset

See `unknown_login`.

### untrusted_set_sender

Type: address list, expanded
Default: unset

When an untrusted user submits a message to Exim using the standard input, Exim normally creates an envelope sender address from the user's login and the default qualification domain. Data from the -f option (for setting envelope senders on non-SMTP messages) or the SMTP MAIL command (if -bs or -bS is used) is ignored.

However, untrusted users are permitted to set an empty envelope sender address, to declare that a message should never generate any bounces. For example:

      exim -f '<>' user@domain.example

The `untrusted_set_sender` option allows you to permit untrusted users to set other envelope sender addresses in a controlled way. When it is set, untrusted users are allowed to set envelope sender addresses that match any of the patterns in the list. Like all address lists, the string is expanded. The identity of the user is in `$sender_ident`, so you can, for example, restrict users to setting senders that start with their login ids followed by a hyphen by a setting like this:

      untrusted_set_sender = ^$sender_ident-

If you want to allow untrusted users to set envelope sender addresses without restriction, you can use

      untrusted_set_sender = *

The `untrusted_set_sender` option applies to all forms of local input, but only to the setting of the envelope sender. It does not permit untrusted users to use the other options which trusted user can use to override message parameters. Furthermore, it does not stop Exim from removing an existing Sender: header in the message, or from adding a Sender: header if necessary. See `local_sender_retain` and `local_from_check` for ways of overriding these actions. The handling of the Sender: header is also described in section 44.14.

The log line for a message's arrival shows the envelope sender following “<=”. For local messages, the user's login always follows, after “U=”. In -bp displays, and in the Exim monitor, if an untrusted user sets an envelope sender address, the user's login is shown in parentheses after the sender address.

### uucp_from_pattern

Type: string
Default: see below

Some applications that pass messages to an MTA via a command line interface use an initial line starting with “From” to pass the envelope sender. In particular, this is used by UUCP software. Exim recognizes such a line by means of a regular expression that is set in `uucp_from_pattern`. When the pattern matches, the sender address is constructed by expanding the contents of `uucp_from_sender`, provided that the caller of Exim is a trusted user. The default pattern recognizes lines in the following two forms:

         From ph10 Fri Jan  5 12:35 GMT 1996
         From ph10 Fri, 7 Jan 97 14:00:00 GMT

The pattern can be seen by running

      exim -bP uucp_from_pattern

It checks only up to the hours and minutes, and allows for a 2-digit or 4-digit year in the second case. The first word after “From” is matched in the regular expression by a parenthesized subpattern. The default value for `uucp_from_sender` is “$1”, which therefore just uses this first word (“ph10” in the example above) as the message's sender. See also `ignore_fromline_hosts`.

### uucp_from_sender

Type: string, expanded
Default: `$1`

See `uucp_from_pattern` above.

### warn_message_file

Type: string
Default: unset

This option defines a template file containing paragraphs of text to be used for constructing the warning message which is sent by Exim when a message has been on the queue for a specified amount of time, as specified by `delay_warning`. Details of the file's contents are given in chapter 41. See also `bounce_message_file`.

### write_rejectlog

Type: boolean
Default: true

If this option is set false, Exim no longer writes anything to the reject log. See chapter 45 for details of what Exim writes to its logs.

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