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Postfix Documentation
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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

Before the cleanup(8) daemon runs an address through any address mapping lookup table, it first rewrites the address to the standard "[email protected]" 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:[email protected]" to "[email protected]"

In case you wonder what this is, the address form above is called a route address, and specifies that mail for "[email protected]" 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 "[email protected]"

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 "[email protected]"

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%[email protected]".

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 "[email protected]$ 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 "[email protected]".

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 "[email protected]$myorigin" to "[email protected]$ myhostname". See also the "delivering some users locally" section in the STANDARD_CONFIGURATION_README document.

Rewrite "[email protected]" to "[email protected]$ mydomain"

This feature is controlled by the boolean append_dot_mydomain parameter (default: 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 "[email protected]" to "[email protected]" (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:
    :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:
    :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:
    :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.

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:
    :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.

Postfix Documentation
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