# Postfix Address Rewriting

## Postfix address rewriting purpose

Address rewriting is at the heart of the Postfix mail system. Postfix rewrites addresses for many different purposes. Some are merely cosmetic, and some are necessary to deliver correctly formatted mail to the correct destination. Examples of address rewriting in Postfix are:

- Transform an incomplete address into a complete address. For example, transform "username" into "username@example.com", or transform "username@hostname" into "username@hostname.example.com".

- Replace an address by an equivalent address. For example, replace "username@example.com" by "firstname.lastname@example.com" when sending mail, and do the reverse transformation when receiving mail.

- Replace an internal address by an external address. For example, replace "username@localdomain.local" by "isp-account@isp.example" when sending mail from a home computer to the Internet.

- Replace an address by multiple addresses. For example, replace the address of an alias by the addresses listed under that alias.

- Determine how and where to deliver mail for a specific address. For example, deliver mail for "username@example.com" with the smtp(8) delivery agent, to the hosts that are listed in the DNS as the mail servers for the domain "example.com".

Although Postfix currently has no address rewriting language, it can do surprisingly powerful address manipulation via table lookup. Postfix typically uses lookup tables with fixed strings to map one address to one or multiple addresses, and typically uses regular expressions to map multiple addresses to one or multiple addresses. Fixed-string lookup tables may be in the form of local files, or in the form of NIS, LDAP or SQL databases. The DATABASE_README document gives an introduction to Postfix lookup tables.

Topics covered in this document:

- To rewrite message headers or not, or to label as invalid
- Postfix address rewriting overview
- Address rewriting when mail is received
  - Rewrite addresses to standard form
  - Canonical address mapping
  - Address masquerading
  - Automatic BCC recipients
  - Virtual aliasing
- Address rewriting when mail is delivered
  - Resolve address to (transport, next-hop destination)
  - Relocated users table
- Address rewriting with remote delivery
  - Generic mapping for outgoing SMTP mail
- Address rewriting with local delivery
  - Local alias database
  - Local per-user .forward files
  - Local catch-all address
- Debugging your address manipulations

## To rewrite message headers or not, or to label as invalid

Postfix versions 2.1 and earlier always rewrite message header addresses, and append Postfix's own domain information to addresses that Postfix considers incomplete. While rewriting message header addresses is OK for mail with a local origin, it is undesirable for remote mail:

- Message header address rewriting is frowned upon by mail standards,
- Appending Postfix's own domain produces incorrect results with some incomplete addresses,
- Appending Postfix's own domain sometimes creates the appearance that spam is sent by local users.

Postfix versions 2.2 give you the option to either not rewrite message headers from remote SMTP clients at all, or to label incomplete addresses in such message headers as invalid. Here is how it works:

- Postfix always rewrites message headers from local SMTP clients and from the Postfix sendmail command, and appends its own domain to incomplete addresses. The local_header_rewrite_clients parameter controls what SMTP clients Postfix considers local (by default, only local network interface addresses).
- Postfix never rewrites message header addresses from remote SMTP clients when the remote_header_rewrite_domain parameter value is empty (the default setting).
- Otherwise, Postfix rewrites message headers from remote SMTP clients, and appends the remote_header_rewrite_domain value to incomplete addresses. This feature can be used to append a reserved domain such as "domain.invalid", so that incomplete addresses cannot be mistaken for local addresses.

## Postfix address rewriting overview

The figure below zooms in on those parts of Postfix that are most involved with address rewriting activity. See the OVERVIEW document for an overview of the complete Postfix architecture. Names followed by a number are Postfix daemon programs, while unnumbered names represent Postfix queues or internal sources of mail messages.

> | trivial- rewrite(8) (std form) |  | trivial- rewrite(8) (resolve)
> ^ `|` | `| v`
> ^ `|` | `| v`
> smtpd(8) | `>-` | cleanup(8) | `->` | incoming | `->` | active | `->` | qmgr(8) | `-<` | smtp(8)
> qmqpd(8) | lmtp(8)
> pickup(8) | local(8)
> | ^ `|` |  |
> ^ `|` | `| v`
> | bounces forwarding notices |  | deferred |

The table below summarizes all Postfix address manipulations. If you're reading this document for the first time, skip forward to "Address rewriting when mail is received". Once you've finished reading the remainder of this document, the table will help you to quickly find what you need.

> Address manipulation | Scope | Daemon | Turn-on controls | Turn-off controls
> --- | --- | --- | --- | ---
> Rewrite addresses to standard form | all mail | trivial- rewrite(8) | append_at_myorigin, append_dot_mydomain, swap_bangpath, allow_percent_hack | local_header_rewrite_clients, remote_header_rewrite_domain
> Canonical address mapping | all mail | cleanup(8) | canonical_maps | receive_override_options, local_header_rewrite_clients, remote_header_rewrite_domain
> Address masquerading | all mail | cleanup(8) | masquerade_domains | receive_override_options, local_header_rewrite_clients, remote_header_rewrite_domain
> Automatic BCC recipients | new mail | cleanup(8) | always_bcc, sender_bcc_maps, recipient_bcc_maps | receive_override_options
> Virtual aliasing | all mail | cleanup(8) | virtual_alias_maps | receive_override_options
> Resolve address to (transport, next-hop destination) | all mail | trivial- rewrite(8) | local_transport, virtual_transport, relay_transport, default_transport, relayhost, sender_dependent_relayhost_maps, sender_dependent_default_transport_maps | content_filter
> Relocated users table | all mail | trivial- rewrite(8) | relocated_maps | none
> Generic mapping table | outgoing SMTP mail | smtp(8) | smtp_generic_maps | none
> Local alias database | local mail only | local(8) | alias_maps | none
> Local per-user .forward files | local mail only | local(8) | forward_path | none
> Local catch-all address | local mail only | local(8) | luser_relay | none

## Address rewriting when mail is received

The cleanup(8) server receives mail from outside of Postfix as well as mail from internal sources such as forwarded mail, undeliverable mail that is bounced to the sender, and postmaster notifications about problems with the mail system.

The cleanup(8) server transforms the sender, recipients and message content into a standard form before writing it to an incoming queue file. The server cleans up sender and recipient addresses in message headers and in the envelope, adds missing message headers such as From: or Date: that are required by mail standards, and removes message headers such as Bcc: that should not be present. The cleanup(8) server delegates the more complex address manipulations to the trivial-rewrite(8) server as described later in this document.

Address manipulations at this stage are:

- Rewrite addresses to standard form
- Canonical address mapping
- Address masquerading
- Automatic BCC recipients
- Virtual aliasing

### Rewrite addresses to standard form

Before the cleanup(8) daemon runs an address through any address mapping lookup table, it first rewrites the address to the standard "user@fully.qualified.domain" form, by sending the address to the trivial-rewrite(8) daemon. The purpose of rewriting to standard form is to reduce the number of entries needed in lookup tables.

The Postfix trivial-rewrite(8) daemon implements the following hard-coded address manipulations:

> Rewrite "@hosta,@hostb:user@site" to "user@site"

>   In case you wonder what this is, the address form above is called a route address, and specifies that mail for "user@site" be delivered via "hosta" and "hostb". Usage of this form has been deprecated for a long time. Postfix has no ability to handle route addresses, other than to strip off the route part.

>   NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non-empty value. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

> Rewrite "site!user" to "user@site"

>   This feature is controlled by the boolean swap_bangpath parameter (default: yes). The purpose is to rewrite UUCP-style addresses to domain style. This is useful only when you receive mail via UUCP, but it probably does not hurt otherwise.

>   NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non-empty value. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

> Rewrite "user%domain" to "user@domain"

>   This feature is controlled by the boolean allow_percent_hack parameter (default: yes). Typically, this is used in order to deal with monstrosities such as "user%domain@otherdomain".

>   NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non-empty value. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

> Rewrite "user" to "user@$myorigin"

>   This feature is controlled by the boolean append_at_myorigin parameter (default: yes). You should never turn off this feature, because a lot of Postfix components expect that all addresses have the form "user@domain".

>   NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter; otherwise they append the domain name specified with the remote_header_rewrite_domain configuration parameter, if one is specified. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

>   If your machine is not the main machine for $myorigin and you wish to have some users delivered locally without going via that main machine, make an entry in the virtual alias table that redirects "user@$myorigin" to "user@$myhostname". See also the "delivering some users locally" section in the STANDARD_CONFIGURATION_README document.

> Rewrite "user@host" to "user@host.$mydomain"

>   This feature is controlled by the boolean append_dot_mydomain parameter (default: Postfix ≥ 3.0: no, Postfix < 3.0: yes). The purpose is to get consistent treatment of different forms of the same hostname.

>   NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter; otherwise they append the domain name specified with the remote_header_rewrite_domain configuration parameter, if one is specified. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

>   Some will argue that rewriting "host" to "host.domain" is bad. That is why it can be turned off. Others like the convenience of having Postfix's own domain appended automatically.

> Rewrite "user@site." to "user@site" (without the trailing dot).

>   A single trailing dot is silently removed. However, an address that ends in multiple dots will be rejected as an invalid address.

>   NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non-empty value. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

### Canonical address mapping

The cleanup(8) daemon uses the canonical(5) tables to rewrite addresses in message envelopes and in message headers. By default all header and envelope addresses are rewritten; this is controlled with the canonical_classes configuration parameter.

NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non-empty value. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

Address rewriting is done for local and remote addresses. The mapping is useful to replace login names by "Firstname.Lastname" style addresses, or to clean up invalid domains in mail addresses produced by legacy mail systems.

Canonical mapping is disabled by default. To enable, edit the canonical_maps parameter in the main.cf file and specify one or more lookup tables, separated by whitespace or commas.

Example:

>     /etc/postfix/main.cf:
>         canonical_maps = hash:/etc/postfix/canonical

>     /etc/postfix/canonical:
>         wietse        Wietse.Venema

For static mappings as shown above, lookup tables such as hash:, ldap:, mysql: or pgsql: are sufficient. For dynamic mappings you can use regular expression tables. This requires that you become intimately familiar with the ideas expressed in regexp_table(5), pcre_table(5) and canonical(5).

In addition to the canonical maps which are applied to both sender and recipient addresses, you can specify canonical maps that are applied only to sender addresses or to recipient addresses.

Example:

>     /etc/postfix/main.cf:
>         sender_canonical_maps = hash:/etc/postfix/sender_canonical
>         recipient_canonical_maps = hash:/etc/postfix/recipient_canonical

The sender and recipient canonical maps are applied before the common canonical maps. The sender_canonical_classes and recipient_canonical_classes parameters control what addresses are subject to sender_canonical_maps and recipient_canonical_maps mappings, respectively.

Sender-specific rewriting is useful when you want to rewrite ugly sender addresses to pretty ones, and still want to be able to send mail to the those ugly address without creating a mailer loop.

Canonical mapping can be turned off selectively for mail received by smtpd(8), qmqpd(8), or pickup(8), by overriding main.cf settings in the master.cf file. This feature is available in Postfix version 2.1 and later.

Example:

>     /etc/postfix/master.cf:
>         127.0.0.1:10026    inet  n      -      n      -      -     smtpd
>             -o receive_override_options=no_address_mappings

Note: do not specify whitespace around the "=" here.

### Address masquerading

Address masquerading is a method to hide hosts inside a domain behind their mail gateway, and to make it appear as if the mail comes from the gateway itself, instead of from individual machines.

NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non-empty value. To get the behavior before Postfix 2.2, specify "local_header_rewrite_clients = static:all".

Address masquerading is disabled by default, and is implemented by the cleanup(8) server. To enable, edit the masquerade_domains parameter in the main.cf file and specify one or more domain names separated by whitespace or commas. When Postfix tries to masquerade a domain, it processes the list from left to right, and processing stops at the first match.

Example:

>     /etc/postfix/main.cf:
>         masquerade_domains = foo.example.com example.com

strips "any.thing.foo.example.com" to "foo.example.com", but strips "any.thing.else.example.com" to "example.com".

A domain name prefixed with "`!`" means do not masquerade this domain or its subdomains:

>     /etc/postfix/main.cf:
>         masquerade_domains = !foo.example.com example.com

does not change "any.thing.foo.example.com" and "foo.example.com", but strips "any.thing.else.example.com" to "example.com".

The masquerade_exceptions configuration parameter specifies what user names should not be subjected to address masquerading. Specify one or more user names separated by whitespace or commas.

Example:

>     /etc/postfix/main.cf:
>         masquerade_exceptions = root

By default, Postfix makes no exceptions.

Subtle point: by default, address masquerading is applied only to message headers and to envelope sender addresses, but not to envelope recipients. This allows you to use address masquerading on a mail gateway machine, while still being able to forward mail from outside to users on individual machines.

In order to subject envelope recipient addresses to masquerading, too, specify (Postfix version 1.1 and later):

>     /etc/postfix/main.cf:
>         masquerade_classes = envelope_sender, envelope_recipient,
>             header_sender, header_recipient

If you rewrite the envelope recipient like this, Postfix will no longer be able to send mail to individual machines.

Address masquerading can be turned off selectively for mail received by smtpd(8), qmqpd(8), or pickup(8), by overriding main.cf settings in the master.cf file. This feature is available in Postfix version 2.1 and later.

Example:

>     /etc/postfix/master.cf:
>         127.0.0.1:10026    inet  n      -      n      -      -     smtpd
>             -o receive_override_options=no_address_mappings

Note: do not specify whitespace around the "=" here.

### Automatic BCC recipients

After applying the canonical and masquerade mappings, the cleanup(8) daemon can generate optional BCC (blind carbon-copy) recipients. Postfix provides three mechanisms:

> always_bcc = address
>   Deliver a copy of all mail to the specified address. In Postfix versions before 2.1, this feature is implemented by smtpd(8), qmqpd(8), or pickup(8).
> sender_bcc_maps = type:table
>   Search the specified "type:table" lookup table with the envelope sender address for an automatic BCC address. This feature is available in Postfix 2.1 and later.
> recipient_bcc_maps = type:table
>   Search the specified "type:table" lookup table with the envelope recipient address for an automatic BCC address. This feature is available in Postfix 2.1 and later.

Note: automatic BCC recipients are produced only for new mail. To avoid mailer loops, automatic BCC recipients are not generated for mail that Postfix forwards internally, nor for mail that Postfix generates itself.

Automatic BCC recipients (including always_bcc) can be turned off selectively for mail received by smtpd(8), qmqpd(8), or pickup(8), by overriding main.cf settings in the master.cf file. This feature is available in Postfix version 2.1 and later.

Example:

>     /etc/postfix/master.cf:
>         127.0.0.1:10026    inet  n      -      n      -      -     smtpd
>             -o receive_override_options=no_address_mappings

Note: do not specify whitespace around the "=" here.

### Virtual aliasing

Before writing the recipients to the queue file, the cleanup(8) daemon uses the optional virtual(5) alias tables to redirect mail for recipients. The mapping affects only envelope recipient addresses; it has no effect on message headers or envelope sender addresses. Virtual alias lookups are useful to redirect mail for virtual alias domains to real user mailboxes, and to redirect mail for domains that no longer exist. Virtual alias lookups can also be used to transform " Firstname.Lastname " back into UNIX login names, although it seems that local aliases may be a more appropriate vehicle. See the VIRTUAL_README document for an overview of methods to host virtual domains with Postfix.

Note: virtual aliasing (virtual_alias_maps) applies to all recipients: local(8), virtual, and remote. This is unlike local aliasing (alias_maps) which applies only to local(8) recipients.

Virtual aliasing is disabled by default. To enable, edit the virtual_alias_maps parameter in the main.cf file and specify one or more lookup tables, separated by whitespace or commas.

Example:

>     /etc/postfix/main.cf:
>         virtual_alias_maps = hash:/etc/postfix/virtual

>     /etc/postfix/virtual:
>         Wietse.Venema        wietse

Addresses found in virtual alias maps are subjected to another iteration of virtual aliasing, but are not subjected to canonical mapping, in order to avoid loops.

For static mappings as shown above, lookup tables such as hash:, ldap:, mysql: or pgsql: are sufficient. For dynamic mappings you can use regular expression tables. This requires that you become intimately familiar with the ideas expressed in regexp_table(5), pcre_table(5) and virtual(5).

Virtual aliasing can be turned off selectively for mail received by smtpd(8), qmqpd(8), or pickup(8), by overriding main.cf settings in the master.cf file. This feature is available in Postfix version 2.1 and later.

Example:

>     /etc/postfix/master.cf:
>         127.0.0.1:10026    inet  n      -      n      -      -     smtpd
>             -o receive_override_options=no_address_mappings

Note: do not specify whitespace around the "=" here.

At this point the message is ready to be stored into the Postfix incoming queue.

## Address rewriting when mail is delivered

The Postfix queue manager sorts mail according to its destination and gives it to Postfix delivery agents such as local(8), smtp(8), or lmtp(8). Just like the cleanup(8) server, the Postfix queue manager delegates the more complex address manipulations to the trivial-rewrite(8) server.

Address manipulations at this stage are:

- Resolve address to (transport, next-hop destination)
- Relocated users table

Each Postfix delivery agent tries to deliver the mail to its destination, while encapsulating the sender, recipients, and message content according to the rules of the SMTP, LMTP, etc. protocol. When mail cannot be delivered, it is either returned to the sender or moved to the deferred queue and tried again later.

Address manipulations when mail is delivered via the smtp(8) delivery agent:

- Generic mapping for outgoing SMTP mail

Address manipulations when mail is delivered via the local(8) delivery agent:

- Local alias database
- Local per-user .forward files
- Local catch-all address

The remainder of this document presents each address manipulation step in more detail, with specific examples or with pointers to documentation with examples.

### Resolve address to (transport, next-hop destination)

The Postfix qmgr(8) queue manager selects new mail from the incoming queue or old mail from the deferred queue. First it looks for overrides:

- The REDIRECT action in access(5), header_checks(5) or body_checks(5) overrides all recipients of the message, and overrides a content_filter setting, and FILTER action in access(5), header_checks(5) or body_checks(5). The REDIRECT action was implemented as a short-cut to retaliate for abuse.

- A content_filter setting and FILTER action in access(5), header_checks(5) or body_checks(5) provide their own (transport, next-hop destination) information. This bypasses all the steps that are described in the remainder of this section.

When there is no content filter override, the qmgr(8) queue manager asks the trivial-rewrite(8) address rewriting and resolving daemon for each recipient how to deliver it (which message delivery transport) and where to deliver it (what next-hop destination).

As of version 2.0, Postfix distinguishes four major domain classes. Each class has its own list of recipient domain names, and each class has its own delivery method, as shown in the table below. See the ADDRESS_CLASS_README document for the fine details. Postfix versions before 2.0 only distinguish between local delivery and everything else.

Note that the table does not match recipients against virtual_alias_domains. The reason is that all valid recipients in a virtual alias domain must be aliased to an address in a different domain. All other recipients in a virtual alias domain are by definition undeliverable, and do not need to be considered here.

> Domain class | Recipient domain match | Delivery method | Availability
> --- | --- | --- | ---
> Local | mydestination, inet_interfaces, proxy_interfaces | local_transport | Postfix 1.0
> Virtual mailbox | virtual_mailbox_domains | virtual_transport | Postfix 2.0
> Relay | relay_domains | relay_transport | Postfix 2.0
> Default | none | default_transport | Postfix 1.0

The delivery methods in the above table may include a next-hop destination in addition to a delivery transport. This may override the next-hop destination that is by default taken from the recipient domain.

Over time, features have been added to override the above transport and/or next-hop destination information. The following table lists where a transport or next-hop destination may be taken from, depending on the recipient domain class.

> Domain class | Transport sources (in order of descending precedence) | Next hop sources (in order of descending precedence)
> --- | --- | ---
> Local | transport_maps, local_transport | transport_maps, local_transport, recipient domain
> Virtual mailbox | transport_maps, virtual_transport | transport_maps, virtual_transport, recipient domain
> Relay | transport_maps, relay_transport | transport_maps, relay_transport, sender_dependent_relayhost_maps, relayhost, recipient domain
> Default | transport_maps, sender_dependent_default_transport_maps, default_transport | transport_maps, sender_dependent_default_transport_maps, default_transport, sender_dependent_relayhost_maps, relayhost, recipient domain

### Relocated users table

Next, the trivial-rewrite(8) address rewriting and resolving daemon runs each recipient through the relocated(5) database. This table provides information on how to reach users that no longer have an account, or what to do with mail for entire domains that no longer exist. When mail is sent to an address that is listed in this table, the message is returned to the sender with an informative message.

The relocated(5) database is searched after transport(5) table lookups, in anticipation of transport(5) tables that can replace one recipient address by a different one.

Lookups of relocated users are disabled by default. To enable, edit the relocated_maps parameter in the main.cf file and specify one or more lookup tables, separated by whitespace or commas.

Example:

>     /etc/postfix/main.cf:
>         relocated_maps = hash:/etc/postfix/relocated

>     /etc/postfix/relocated:
>         username@example.com      otheruser@elsewhere.tld

As of Postfix version 2, mail for a relocated user will be rejected by the SMTP server with the reason "user has moved to otheruser@elsewhere.tld". Older Postfix versions will receive the mail first, and then return it to the sender as undeliverable, with the same reason.

### Generic mapping for outgoing SMTP mail

Some hosts have no valid Internet domain name, and instead use a name such as localdomain.local. This can be a problem when you want to send mail over the Internet, because many mail servers reject mail addresses with invalid domain names.

With the smtp_generic_maps parameter you can specify generic(5) lookup tables that replace local mail addresses by valid Internet addresses when mail leaves the machine via SMTP. The generic(5) mapping replaces envelope and header addresses, and is non-recursive. It does not happen when you send mail between addresses on the local machine.

This feature is available in Postfix version 2.2 and later.

Example:

>     /etc/postfix/main.cf:
>         smtp_generic_maps = hash:/etc/postfix/generic

>     /etc/postfix/generic:
>         his@localdomain.local               hisaccount@hisisp.example
>         her@localdomain.local               heraccount@herisp.example
>         @localdomain.local                  hisaccount+local@hisisp.example

When mail is sent to a remote host via SMTP, this replaces his@localdomain.local by his ISP mail address, replaces her@localdomain.local by her ISP mail address, and replaces other local addresses by his ISP account, with an address extension of +local (this example assumes that the ISP supports "+" style address extensions).

### Local alias database

When mail is to be delivered locally, the local(8) delivery agent runs each local recipient name through the aliases(5) database. The mapping does not affect addresses in message headers. Local aliases are typically used to implement distribution lists, or to direct mail for standard aliases such as postmaster to real people. The table can also be used to map "Firstname.Lastname" addresses to login names.

Note: local aliasing (alias_maps) applies only to local(8) recipients. This is unlike virtual aliasing (virtual_alias_maps) which applies to all recipients: local(8), virtual, and remote.

Alias lookups are enabled by default. The default configuration depends on the operating system environment, but it is typically one of the following:

>     /etc/postfix/main.cf:
>         alias_maps = hash:/etc/aliases
>         alias_maps = dbm:/etc/aliases, nis:mail.aliases

The pathname of the alias database file is controlled with the alias_database configuration parameter. The value is system dependent. Usually it is one of the following:

>     /etc/postfix/main.cf:
>         alias_database = hash:/etc/aliases (4.4BSD, LINUX)
>         alias_database = dbm:/etc/aliases (4.3BSD, SYSV<4)
>         alias_database = dbm:/etc/mail/aliases (SYSV4)

An aliases(5) file can specify that mail should be delivered to a local file, or to a command that receives the message in the standard input stream. For security reasons, deliveries to command and file destinations are performed with the rights of the alias database owner. A default userid, default_privs, is used for deliveries to commands or files in "root"-owned aliases.

### Local per-user .forward files

With delivery via the local(8) delivery agent, users can control their own mail delivery by specifying destinations in a file called .forward in their home directories. The syntax of these files is the same as with the local aliases(5) file, except that the left-hand side of the alias (lookup key and colon) are not present.

### Local catch-all address

When the local(8) delivery agent finds that a message recipient does not exist, the message is normally returned to the sender ("user unknown"). Sometimes it is desirable to forward mail for non-existing recipients to another machine. For this purpose you can specify an alternative destination with the luser_relay configuration parameter.

Alternatively, mail for non-existent recipients can be delegated to an entirely different message transport, as specified with the fallback_transport configuration parameter. For details, see the local(8) delivery agent documentation.

Note: if you use the luser_relay feature in order to receive mail for non-UNIX accounts, then you must specify:

>     /etc/postfix/main.cf:
>         local_recipient_maps =

(i.e. empty) in the main.cf file, otherwise the Postfix SMTP server will reject mail for non-UNIX accounts with "User unknown in local recipient table". See the LOCAL_RECIPIENT_README file for more information on this.

luser_relay can specify one address. It is subjected to "$name" expansions. Examples:

> $user@other.host

>   The bare username, without address extension, is prepended to "@other.host". For example, mail for "username+foo" is sent to "username@other.host".

> $local@other.host

>   The entire original recipient localpart, including address extension, is prepended to "@other.host". For example, mail for "username+foo" is sent to "username+foo@other.host".

> sysadmin+$user

>   The bare username, without address extension, is appended to "sysadmin". For example, mail for "username+foo" is sent to "sysadmin+username".

> sysadmin+$local

>   The entire original recipient localpart, including address extension, is appended to "sysadmin". For example, mail for "username+foo" is sent to "sysadmin+username+foo".

## Debugging your address manipulations

Postfix version 2.1 and later can produce mail delivery reports for debugging purposes. These reports not only show sender/recipient addresses after address rewriting and alias expansion or forwarding, they also show information about delivery to mailbox, delivery to non-Postfix command, responses from remote SMTP servers, and so on.

Postfix can produce two types of mail delivery reports for debugging:

- What-if: report what would happen, but do not actually deliver mail. This mode of operation is requested with:

      $ /usr/sbin/sendmail -bv address...
      Mail Delivery Status Report will be mailed to <your login name>.

- What happened: deliver mail and report successes and/or failures, including replies from remote SMTP servers. This mode of operation is requested with:

      $ /usr/sbin/sendmail -v address...
      Mail Delivery Status Report will be mailed to <your login name>.

These reports contain information that is generated by Postfix delivery agents. Since these run as daemon processes and do not interact with users directly, the result is sent as mail to the sender of the test message. The format of these reports is practically identical to that of ordinary non-delivery notifications.

As an example, below is the delivery report that is produced with the command "sendmail -bv postfix-users@postfix.org". The first part of the report contains human-readable text. In this case, mail would be delivered via mail.cloud9.net, and the SMTP server replies with "250 Ok". Other reports may show delivery to mailbox, or delivery to non-Postfix command.

>     Content-Description: Notification
>     Content-Type: text/plain

>     This is the mail system at host spike.porcupine.org.

>     Enclosed is the mail delivery report that you requested.

>                             The mail system

>     <postfix-users@postfix.org>: delivery via mail.cloud9.net[168.100.1.4]: 250 2.1.5 Ok

The second part of the report is in machine-readable form, and includes the following information:

- The envelope sender address (wietse@porcupine.org).
- The envelope recipient address (postfix-users@postfix.org). If the recipient address was changed by Postfix then Postfix also includes the original recipient address.
- The delivery status.

Some details depend on Postfix version. The example below is for Postfix version 2.3 and later.

>     Content-Description: Delivery report
>     Content-Type: message/delivery-status

>     Reporting-MTA: dns; spike.porcupine.org
>     X-Postfix-Queue-ID: 84863BC0E5
>     X-Postfix-Sender: rfc822; wietse@porcupine.org
>     Arrival-Date: Sun, 26 Nov 2006 17:01:01 -0500 (EST)

>     Final-Recipient: rfc822; postfix-users@postfix.org
>     Action: deliverable
>     Status: 2.1.5
>     Remote-MTA: dns; mail.cloud9.net
>     Diagnostic-Code: smtp; 250 2.1.5 Ok

The third part of the report contains the message that Postfix would have delivered, including From: and To: message headers, so that you can see any effects of address rewriting on those. Mail submitted with "sendmail -bv" has no body content so none is shown in the example below.

>     Content-Description: Message
>     Content-Type: message/rfc822

>     Received: by spike.porcupine.org (Postfix, from userid 1001)
>             id 84863BC0E5; Sun, 26 Nov 2006 17:01:01 -0500 (EST)
>     Subject: probe
>     To: postfix-users@postfix.org
>     Message-Id: <20061126220101.84863BC0E5@spike.porcupine.org>
>     Date: Sun, 26 Nov 2006 17:01:01 -0500 (EST)
>     From: wietse@porcupine.org (Wietse Venema)