Exim 4.40 Specification chapter 22

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# 22. The redirect router

The redirect router handles several kinds of address redirection. Its most common uses are for resolving local part aliases from a central alias file (usually called /etc/aliases) and for handling users' personal .forward files, but it has many other potential uses. The incoming address can be redirected in several different ways:

- It can be replaced by one or more new addresses which are themselves routed independently.

- It can be routed to be delivered to a given file or directory.

- It can be routed to be delivered to a specified pipe command.

- It can cause an automatic reply to be generated.

- It can be forced to fail, with a custom error message.

- It can be temporarily deferred.

- It can be discarded.

The generic `transport` option must not be set for redirect routers. However, there are some private options which define transports for delivery to files and pipes, and for generating autoreplies. See the `file_transport`, `pipe_transport` and `reply_transport` descriptions below.

## 22.1. Redirection data

The router operates by interpreting a text string which it obtains either by expanding the contents of the `data` option, or by reading the entire contents of a file whose name is given in the `file` option. These two options are mutually exclusive. The first is commonly used for handling system aliases, in a configuration like this:

      system_aliases:
        driver = redirect
        data = ${lookup{$local_part}lsearch{/etc/aliases}}

If the lookup fails, the expanded string in this example is empty. When the expansion of `data` results in an empty string, the router declines. A forced expansion failure also causes the router to decline; other expansion failures cause delivery to be deferred.

A configuration using `file` is commonly used for handling users' .forward files, like this:

      userforward:
        driver = redirect
        check_local_user
        file = $home/.forward
        no_verify

If the file does not exist, or causes no action to be taken (for example, it is empty or consists only of comments), the router declines. Warning: This is not the case when the file contains syntactically valid items that happen to yield empty addresses, for example, items containing only RFC 2822 address comments.

## 22.2. Forward files and address verification

It is usual to set `no_verify` on redirect routers which handle users' .forward files, as in the example above. There are two reasons for this:

- When Exim is receiving an incoming SMTP message from a remote host, it is running under the Exim uid, not as root. No additional groups are set up, even if the Exim uid is a member of other groups (that is, the initgroups() function is not run). Exim is unable to change uid to read the file as the user, and it may not be able to read it as the Exim user. So in practice the router may not be able to operate.

- However, even when the router can operate, the existence of a .forward file is unimportant when verifying an address. What should be checked is whether the local part is a valid user name or not. Cutting out the redirection processing saves some resources.

## 22.3. Interpreting redirection data

The contents of the data string, whether obtained from `data` or `file`, can be interpreted in two different ways:

- If the `allow_filter` option is set true, and the data begins with the text “#Exim filter” or “#Sieve filter”, it is interpreted as a list of filtering instructions in the form of an Exim or Sieve filter file, respectively. Details of the syntax and semantics of filter files are described in a separate document entitled Exim's interfaces to mail filtering; this document is intended for use by end users.

- Otherwise, the data must be a comma-separated list of redirection items, as described in the next section.

When a message is redirected to a file (a “mail folder”), the file name given in a non-filter redirection list must always be an absolute path. A filter may generate a relative path – how this is handled depends on the transport's configuration. See section 26.1 for a discussion of this issue for the appendfile transport.

## 22.4. Items in a non-filter redirection list

When the redirection data is not an Exim or Sieve filter, for example, if it comes from a conventional alias or forward file, it consists of a list of addresses, file names, pipe commands, or certain special items (see section 22.6 below). The special items can be individually enabled or disabled by means of options whose names begin with `allow_` or `forbid_`, depending on their default values. The items in the list are separated by commas or newlines. If a comma is required in an item, the entire item must be enclosed in double quotes.

Lines starting with a # character are comments, and are ignored, and # may also appear following a comma, in which case everything between the # and the next newline character is ignored.

If an item is entirely enclosed in double quotes, these are removed. Otherwise double quotes are retained because some forms of mail address require their use (but never to enclose the entire address). In the following description, “item” refers to what remains after any surrounding double quotes have been removed.

Warning: If you use an Exim expansion to construct a redirection address, and the expansion contains a reference to `$local_part`, you should make use of the `quote` expansion operator, in case the local part contains special characters. For example, to redirect all mail for the domain obsolete.example, retaining the existing local part, you could use this setting:

      data = ${quote:$local_part}@newdomain.example

## 22.5. Redirecting to a local mailbox

A redirection item may safely be the same as the address currently under consideration. This does not cause a routing loop, because a router is automatically skipped if any ancestor of the address that is being processed is the same as the current address and was processed by the current router. Such an address is therefore passed to the following routers, so it is handled as if there were no redirection. When making this loop-avoidance test, the complete local part, including any prefix or suffix, is used.

Specifying the same local part without a domain is a common usage in personal filter files when the user wants to have messages delivered to the local mailbox and also forwarded elsewhere. For example, the user whose login is cleo might have a .forward file containing this:

      cleo, cleopatra@egypt.example

For compatibility with other MTAs, such unqualified local parts may be preceeded by “\”, but this is not a requirement for loop prevention. However, it does make a difference if more than one domain is being handled synonymously.

If an item begins with “\” and the rest of the item parses as a valid RFC 2822 address that does not include a domain, the item is qualified using the domain of the incoming address. In the absence of a leading “\”, unqualified addresses are qualified using the value in `qualify_recipient`, but you can force the incoming domain to be used by setting `qualify_preserve_domain`.

Care must be taken if there are alias names for local users. Consider an MTA handling a single local domain where the system alias file contains:

      Sam.Reman: spqr

Now suppose that Sam (whose login id is spqr) wants to save copies of messages in the local mailbox, and also forward copies elsewhere. He creates this forward file:

      Sam.Reman, spqr@reme.elsewhere.example

With these settings, an incoming message addressed to Sam.Reman fails. The redirect router for system aliases does not process Sam.Reman the second time round, because it has previously routed it, and the following routers presumably cannot handle the alias. The forward file should really contain

      spqr, spqr@reme.elsewhere.example

but because this is such a common error, the `check_ancestor` option (see below) exists to provide a way to get round it. This is normally set on a redirect router that is handling users' .forward files.

## 22.6. Special items in redirection lists

In addition to addresses, the following types of item may appear in redirection lists (that is, in non-filter redirection data):

- An item is treated as a pipe command if it begins with “|” and does not parse as a valid RFC 2822 address that includes a domain. A transport for running the command must be specified by the `pipe_transport` option. Normally, either the router or the transport specifies a user and a group under which to run the delivery. The default is to use the Exim user and group.

  Single or double quotes can be used for enclosing the individual arguments of the pipe command; no interpretation of escapes is done for single quotes. If the command contains a comma character, it is necessary to put the whole item in double quotes, for example:

        "|/some/command ready,steady,go"

  since items in redirection lists are terminated by commas. Do not, however, quote just the command. An item such as

        |"/some/command ready,steady,go"

  is interpreted as a pipe with a rather strange command name, and no arguments.

- An item is interpreted as a path name if it begins with “/” and does not parse as a valid RFC 2822 address that includes a domain. For example,

        /home/world/minbari

  is treated as a file name, but

        /s=molari/o=babylon/@x400gate.way

  is treated as an address. For a file name, a transport must be specified using the `file_transport` option. However, if the generated path name ends with a forward slash character, it is interpreted as a directory name rather than a file name, and `directory_transport` is used instead.

  Normally, either the router or the transport specifies a user and a group under which to run the delivery. The default is to use the Exim user and group. However, if a redirection item is the path /dev/null, delivery to it is bypassed at a high level, and the log entry shows “**bypassed**” instead of a transport name. In this case the user and group are not used.

- If an item is of the form

  `` `:include:<path name>`

  a list of further items is taken from the given file and included at that point. Note: such a file can not be a filter file; it is just an out-of-line addition to the list. The items in the included list are separated by commas or newlines and are not subject to expansion. If this is the first item in an alias list in an lsearch file, a colon must be used to terminate the alias name. This example is incorrect:

        list1    :include:/opt/lists/list1

  It must be given as

        list1:   :include:/opt/lists/list1

- Sometimes you want to throw away mail to a particular local part. Making the `data` option expand to an empty string does not work, because that causes the router to decline. Instead, the alias item

  `` `:blackhole:`

  can be used. It does what its name implies. No delivery is done, and no error message is generated. This has the same effect as specifing /dev/null, but can be independently disabled.

  Warning: If `:blackhole:` appears anywhere in a redirection list, no delivery is done for the original local part, even if other redirection items are present. If you are generating a multi-item list (for example, by reading a database) and need the ability to provide a no-op item, you must use /dev/null.

- An attempt to deliver a particular address can be deferred or forced to fail by redirection items of the form

  `` `:defer:`
  `` or
  `` `:fail:`

  respectively. When a redirection list contains such an item, it applies to the entire redirection; any other items in the list are ignored (:blackhole: is different). Any text following :fail: or :defer: is placed in the error text associated with the failure. For example, an alias file might contain:

        X.Employee:  :fail: Gone away, no forwarding address

  In the case of an address that is being verified from an ACL or as the subject of a VRFY command, the text is included in the SMTP error response by default. In an ACL, an explicitly provided message overrides the default, but the default message is available in the variable `$acl_verify_message` and can therefore be included in a custom message if this is desired. Exim sends a 451 SMTP code for a :defer:, and 550 for :fail:. In non-SMTP cases the text is included in the error message that Exim generates.

  Normally the error text is the rest of the redirection list – a comma does not terminate it – but a newline does act as a terminator. Newlines are not normally present in alias expansions. In lsearch lookups they are removed as part of the continuation process, but they may exist in other kinds of lookup and in :include: files.

  During routing for message delivery (as opposed to verification), a redirection containing :fail: causes an immediate failure of the incoming address, whereas :defer: causes the message to remain on the queue so that a subsequent delivery attempt can happen at a later time. If an address is deferred for too long, it will ultimately fail, because the normal retry rules still apply.

- Sometimes it is useful to use a single-key search type with a default (see chapter 9) to look up aliases. However, there may be a need for exceptions to the default. These can be handled by aliasing them to

        :unknown:

  This differs from :fail: in that it causes the redirect router to decline, whereas :fail: forces routing to fail. A lookup which results in an empty redirection list has the same effect.

## 22.7. Duplicate addresses

Exim removes duplicate addresses from the list to which it is delivering, so as to deliver just one copy to each address. This does not apply to deliveries routed to pipes by different immediate parent addresses, but an indirect aliasing scheme of the type

      pipe:       |/some/command $local_part
      localpart1: pipe
      localpart2: pipe

does not work with a message that is addressed to both local parts, because when the second is aliased to the intermediate local part “pipe” it gets discarded as being the same as a previously handled address. However, a scheme such as

      localpart1: |/some/command $local_part
      localpart2: |/some/command $local_part

does result in two different pipe deliveries, because the immediate parents of the pipes are distinct.

## 22.8. Repeated redirection expansion

When a message cannot be delivered to all of its recipients immediately, leading to two or more delivery attempts, redirection expansion is carried out afresh each time for those addresses whose children were not all previously delivered. If redirection is being used as a mailing list, this can lead to new members of the list receiving copies of old messages. The `one_time` option can be used to avoid this.

## 22.9. Errors in redirection lists

If `skip_syntax_errors` is set, a malformed address that causes a parsing error is skipped, and an entry is written to the main log. This may be useful for mailing lists that are automatically managed. Otherwise, if an error is detected while generating the list of new addresses, the original address is deferred. See also `syntax_errors_to`.

## 22.10. Private options for the redirect router

The private options for the redirect router are as follows:

### allow_defer

Type: boolean
Default: false

Setting this option allows the use of :defer: in non-filter redirection data, or the `defer` command in an Exim filter file.

### allow_fail

Type: boolean
Default: false

If this option is true, the :fail: item can be used in a redirection list, and the `fail` command may be used in a filter file.

### allow_filter

Type: boolean
Default: false

Setting this option allows Exim to interpret redirection data that starts with “#Exim filter” or “#Sieve filter” as a set of filtering instructions. There are some features of Exim filter files that some administrators may wish to lock out; see the `forbid_filter_xxx` options below. The filter is run using the uid and gid set by the generic `user` and `group` options. These take their defaults from the password data if `check_local_user` is set, so in the normal case of users' personal filter files, the filter is run as the relevant user. When `allow_filter` is set true, Exim insists that either `check_local_user` or `user` is set.

### allow_freeze

Type: boolean
Default: false

Setting this option allows the use of the `freeze` command in an Exim filter. This command is more normally encountered in system filters, and is disabled by default for redirection filters because it isn't something you usually want to let ordinary users do.

### check_ancestor

Type: boolean
Default: false

This option is concerned with handling generated addresses that are the same as some address in the list of redirection ancestors of the current address. Although it is turned off by default in the code, it is set in the default configuration file for handling users' .forward files. It is recommended for this use of the redirect router.

When `check_ancestor` is set, if a generated address (including the domain) is the same as any ancestor of the current address, it is replaced by a copy of the current address. This helps in the case where local part A is aliased to B, and B has a .forward file pointing back to A. For example, within a single domain, the local part “Joe.Bloggs” is aliased to “jb” and ~jb/.forward contains:

`` `\Joe.Bloggs, <other item(s)>`

Without the `check_ancestor` setting, either local part (“jb” or “joe.bloggs”) gets processed once by each router and so ends up as it was originally. If “jb” is the real mailbox name, mail to “jb” gets delivered (having been turned into “joe.bloggs” by the .forward file and back to “jb” by the alias), but mail to “joe.bloggs” fails. Setting `check_ancestor` on the redirect router that handles the .forward file prevents it from turning “jb” back into “joe.bloggs” when that was the original address. See also the `repeat_use` option below.

### check_group

Type: boolean
Default: see below

When the `file` option is used, the group owner of the file is checked only when this option is set. The permitted groups are those listed in the `owngroups` option, together with the user's default group if `check_local_user` is set. If the file has the wrong group, routing is deferred. The default setting for this option is true if `check_local_user` is set and the `modemask` option permits the group write bit, or if the `owngroups` option is set. Otherwise it is false, and no group check occurs.

### check_owner

Type: boolean
Default: see below

When the `file` option is used, the owner of the file is checked only when this option is set. If `check_local_user` is set, the local user is permitted; otherwise the owner must be one of those listed in the `owners` option. The default value for this option is true if `check_local_user` or `owners` is set. Otherwise the default is false, and no owner check occurs.

### data

Type: string, expanded
Default: unset

This option is mutually exclusive with `file`. One or other of them must be set, but not both. The contents of `data` are expanded, and then used as the list of forwarding items, or as a set of filtering instructions. If the expansion is forced to fail, or the result is an empty string or a string that has no effect (consists entirely of comments), the router declines.

When filtering instructions are used, the string must begin with “#Exim filter”, and all comments in the string, including this initial one, must be terminated with newline characters. For example:

      data = #Exim filter\n\
             if $h_to: contains Exim then save $home/mail/exim endif

If you are reading the data from a database where newlines cannot be included, you can use the `${sg}` expansion item to turn the escape string of your choice into a newline.

### directory_transport

Type: string, expanded
Default: unset

A redirect router sets up a direct delivery to a directory when a path name ending with a slash is specified as a new “address”. The transport used is specified by this option, which, after expansion, must be the name of a configured transport. This should normally be an appendfile transport.

### file

Type: string, expanded
Default: unset

This option specifies the name of a file that contains the redirection data. It is mutually exclusive with the `data` option. The string is expanded before use; if the expansion is forced to fail, the router declines. Other expansion failures cause delivery to be deferred. The result of a successful expansion must be an absolute path. The entire file is read and used as the redirection data. If the data is an empty string or a string that has no effect (consists entirely of comments), the router declines.

If the attempt to open the file fails with a “does not exist” error, Exim runs a check on the containing directory, unless `ignore_enotdir` is true (see below). If the directory does not appear to exist, delivery is deferred. This can happen when users' .forward files are in NFS-mounted directories, and there is a mount problem. If the containing directory does exist, but the file does not, the router declines.

### file_transport

Type: string, expanded
Default: unset

A redirect router sets up a direct delivery to a file when a path name not ending in a slash is specified as a new “address”. The transport used is specified by this option, which, after expansion, must be the name of a configured transport. This should normally be an appendfile transport. When it is running, the file name is in `$address_file`.

### forbid_blackhole

Type: boolean
Default: false

If this option is true, the :blackhole: item may not appear in a redirection list.

### forbid_file

Type: boolean
Default: false

If this option is true, this router may not generate a new address that specifies delivery to a local file or directory, either from a filter or from a conventional forward file. This option is forced to be true if `one_time` is set. It applies to Sieve filters as well as to Exim filters, but if true, it locks out the Sieve's “keep” facility.

### forbid_filter_existstest

Type: boolean
Default: false

If this option is true, string expansions in Exim filters are not allowed to make use of the `exists` condition.

### forbid_filter_logwrite

Type: boolean
Default: false

If this option is true, use of the logging facility in Exim filters is not permitted. Logging is in any case available only if the filter is being run under some unprivileged uid (which is normally the case for ordinary users' .forward files).

### forbid_filter_lookup

Type: boolean
Default: false

If this option is true, string expansions in Exim filter files are not allowed to make use of `lookup` items.

### forbid_filter_perl

Type: boolean
Default: false

This option is available only if Exim is built with embedded Perl support. If it is true, string expansions in Exim filter files are not allowed to make use of the embedded Perl support.

### forbid_filter_readfile

Type: boolean
Default: false

If this option is true, string expansions in Exim filter files are not allowed to make use of `readfile` items.

### forbid_filter_readsocket

Type: boolean
Default: false

If this option is true, string expansions in Exim filter files are not allowed to make use of `readsocket` items.

### forbid_filter_reply

Type: boolean
Default: false

If this option is true, this router may not generate an automatic reply message. Automatic replies can be generated only from Exim filter files, not from traditional forward files or Sieve filters. This option is forced to be true if `one_time` is set.

### forbid_filter_run

Type: boolean
Default: false

If this option is true, string expansions in Exim filter files are not allowed to make use of `run` items.

### forbid_include

Type: boolean
Default: false

If this option is true, items of the form

`` `:include:<path name>`

are not permitted in non-filter redirection lists.

### forbid_pipe

Type: boolean
Default: false

If this option is true, this router may not generate a new address which specifies delivery to a pipe, either from an Exim filter or from a conventional forward file. This option is forced to be true if `one_time` is set.

### hide_child_in_errmsg

Type: boolean
Default: false

If this option is true, it prevents Exim from quoting a child address if it generates a bounce or delay message for it. Instead it says “an address generated from <the top level address>”. Of course, this applies only to bounces generated locally. If a message is forwarded to another host, its bounce may well quote the generated address.

### ignore_eacces

Type: boolean
Default: false

If this option is set and an attempt to open a redirection file yields the EACCES error (permission denied), the redirect router behaves as if the file did not exist.

### ignore_enotdir

Type: boolean
Default: false

If this option is set and an attempt to open a redirection file yields the ENOTDIR error (something on the path is not a directory), the redirect router behaves as if the file did not exist.

Setting `ignore_enotdir` has another effect as well: When a redirect router that has the `file` option set discovers that the file does not exist (the ENOENT error), it tries to stat() the parent directory, as a check against unmounted NFS directories. If the parent can not be statted, delivery is deferred. However, it seems wrong to do this check when `ignore_enotdir` is set, because that option tells Exim to ignore “something on the path is not a directory” (the ENOTDIR error). This is a confusing area, because it seems that some operating systems give ENOENT where others give ENOTDIR.

### include_directory

Type: string
Default: unset

If this option is set, the path names of any :include: items in a redirection list must start with this directory.

### modemask

Type: octal integer
Default: 022

This specifies mode bits which must not be set for a file specified by the `file` option. If any of the forbidden bits are set, delivery is deferred.

### one_time

Type: boolean
Default: false

Sometimes the fact that Exim re-evaluates aliases and reprocesses redirection files each time it tries to deliver a message causes a problem when one or more of the generated addresses fails be delivered at the first attempt. The problem is not one of duplicate delivery – Exim is clever enough to handle that – but of what happens when the redirection list changes during the time that the message is on Exim's queue. This is particularly true in the case of mailing lists, where new subscribers might receive copies of messages that were posted before they subscribed.

If `one_time` is set and any addresses generated by the router fail to deliver at the first attempt, the failing addresses are added to the message as “top level” addresses, and the parent address that generated them is marked “delivered”. Thus, redirection does not happen again at the next delivery attempt.

Warning 1: This means that any header line addition or removal that is specified by this router would be lost if delivery did not succeed at the first attempt. For this reason, the `headers_add` and `headers_remove` generic options are not permitted when `one_time` is set.

Warning 2: To ensure that the router generates only addresses (as opposed to pipe or file deliveries or auto-replies) `forbid_file`, `forbid_pipe`, and `forbid_filter_reply` are forced to be true when `one_time` is set.

The original top-level address is remembered with each of the generated addresses, and is output in any log messages. However, any intermediate parent addresses are not recorded. This makes a difference to the log only if `all_parents` log selector is set. It is expected that `one_time` will typically be used for mailing lists, where there is normally just one level of expansion.

### owners

Type: string list
Default: unset

This specifies a list of permitted owners for the file specified by `file`. This list is in addition to the local user when `check_local_user` is set. See `check_owner` above.

### owngroups

Type: string list
Default: unset

This specifies a list of permitted groups for the file specified by `file`. The list is in addition to the local user's primary group when `check_local_user` is set. See `check_group` above.

### qualify_domain

Type: string, expanded
Default: unset

If this option is set and an unqualified address (one without a domain) is generated, it is qualified with the domain specified by expanding this string, instead of the global setting in `qualify_recipient`. If the expansion fails, the router declines. If you want to revert to the default, you can have the expansion generate `$qualify_recipient`.

### pipe_transport

Type: string, expanded
Default: unset

A redirect router sets up a direct delivery to a pipe when a string starting with a vertical bar character is specified as a new “address”. The transport used is specified by this option, which, after expansion, must be the name of a configured transport. This should normally be a pipe transport. When the transport is run, the pipe command is in `$address_pipe`.

### qualify_preserve_domain

Type: boolean
Default: false

If this is set and an unqualified address (one without a domain) is generated, it is qualified with the domain of the parent address (the immediately preceding ancestor) instead of the local `qualify_domain` or global `qualify_recipient` value.

### repeat_use

Type: boolean
Default: true

If this option is set false, the router is skipped for a child address that has any ancestor that was routed by this router. This test happens before any of the other preconditions are tested. Exim's default anti-looping rules skip only when the ancestor is the same as the current address. See also `check_ancestor` above and the generic `redirect_router` option.

### reply_transport

Type: string, expanded
Default: unset

A redirect router sets up an automatic reply when a `mail` or `vacation` command is used in a filter file. The transport used is specified by this option, which, after expansion, must be the name of a configured transport. This should normally be an autoreply transport. Other transports are unlikely to do anything sensible or useful.

### rewrite

Type: boolean
Default: true

If this option is set false, addresses generated by the router are not subject to address rewriting. Otherwise, they are treated like new addresses and are rewritten according to the global rewriting rules.

### skip_syntax_errors

Type: boolean
Default: false

If `skip_syntax_errors` is set, syntactically malformed addresses in non-filter redirection data are skipped, and each failing address is logged. If `syntax_errors_to` is set, a message is sent to the address it defines, giving details of the failures. If `syntax_errors_text` is set, its contents are expanded and placed at the head of the error message generated by `syntax_errors_to`. Usually it is appropriate to set `syntax_errors_to` to be the same address as the generic `errors_to` option. The `skip_syntax_errors` option is often used when handling mailing lists.

If all the addresses in a redirection list are skipped because of syntax errors, the router declines to handle the original address, and it is passed to the following routers.

If `skip_syntax_errors` is set when an Exim filter is interpreted, any syntax error in the filter causes filtering to be abandoned without any action being taken. The incident is logged, and the router declines to handle the address, so it is passed to the following routers.

Currently, any syntax errors in a Sieve filter file cause the “keep” action to occur. The values of `skip_syntax_errors`, `syntax_errors_to`, and `syntax_errors_text` are not used.

`skip_syntax_errors` can be used to specify that errors in users' forward lists or filter files should not prevent delivery. The `syntax_errors_to` option, used with an address that does not get redirected, can be used to notify users of these errors, by means of a router like this:

      userforward:
        driver = redirect
        allow_filter
        check_local_user
        file = $home/.forward
        file_transport = address_file
        pipe_transport = address_pipe
        reply_transport = address_reply
        no_verify
        skip_syntax_errors
        syntax_errors_to = real-$local_part@$domain
        syntax_errors_text = \
          This is an automatically generated message. An error has\n\
          been found in your .forward file. Details of the error are\n\
          reported below. While this error persists, you will receive\n\
          a copy of this message for every message that is addressed\n\
          to you. If your .forward file is a filter file, or if it is\n\
          a non-filter file containing no valid forwarding addresses,\n\
          a copy of each incoming message will be put in your normal\n\
          mailbox. If a non-filter file contains at least one valid\n\
          forwarding address, forwarding to the valid addresses will\n\
          happen, and those will be the only deliveries that occur.

You also need a router to ensure that local addresses that are prefixed by `real-` are recognized, but not forwarded or filtered. For example, you could put this immediately before the userforward router:

      real_localuser:
        driver = accept
        check_local_user
        local_part_prefix = real-
        transport = local_delivery

### syntax_errors_text

Type: string, expanded
Default: unset

See `skip_syntax_errors` above.

### syntax_errors_to

Type: string
Default: unset

See `skip_syntax_errors` above.

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