postmap /etc/postfix/transport postmap -q "string" /etc/postfix/transport postmap -q - /etc/postfix/transport <inputfile
This mapping overrides the default transport:nexthop selection that is built into Postfix:
Normally, the transport(5) table is specified as a text file that serves as input to the postmap(1) command. The result, an indexed file in dbm or db format, is used for fast searching by the mail system. Execute the command "postmap /etc/postfix/transport" to rebuild an indexed file after changing the corresponding transport table.
When the table is provided via other means such as NIS, LDAP or SQL, the same lookups are done as for ordinary indexed files.
Alternatively, the table can be provided as a regular-expression map where patterns are given as regular expressions, or lookups can be directed to TCP-based server. In those case, the lookups are done in a slightly different way as described below under "REGULAR EXPRESSION TABLES" or "TCP-BASED TABLES".
The search string is folded to lowercase before database lookup. As of Postfix 2.3, the search string is not case folded with database types such as regexp: or pcre: whose lookup fields can match both upper and lower case.
The input format for the postmap(1) command is as follows:
The pattern specifies an email address, a domain name, or a domain name hierarchy, as described in section "TABLE LOOKUP".
With lookups from indexed files such as DB or DBM, or from networked tables such as NIS, LDAP or SQL, patterns are tried in the order as listed below:
Note 1: the null recipient address is looked up as $empty_address_recipient@$myhostname (default: mailer-daemon@hostname).
The lookup result is of the form transport:nexthop. The transport field specifies a mail delivery transport such as smtp or local. The nexthop field specifies where and how to deliver mail.
The transport field specifies the name of a mail delivery transport (the first name of a mail delivery service entry in the Postfix master.cf file).
The interpretation of the nexthop field is transport dependent. In the case of SMTP, specify a service on a non-default port as host:service, and disable MX (mail exchanger) DNS lookups with [host] or [host]:port. The  form is required when you specify an IP address instead of a hostname.
A null transport and null nexthop result means "do not change": use the delivery transport and nexthop information that would be used when the entire transport table did not exist.
A non-null transport field with a null nexthop field resets the nexthop information to the recipient domain.
In order to deliver internal mail directly, while using a mail relay for all other mail, specify a null entry for internal destinations (do not change the delivery transport or the nexthop information) and specify a wildcard for all other destinations.
my.domain : .my.domain : * smtp:outbound-relay.my.domain
In order to send mail for example.com and its subdomains via the uucp transport to the UUCP host named example:
example.com uucp:example .example.com uucp:example
When no nexthop host name is specified, the destination domain name is used instead. For example, the following directs mail for email@example.com via the slow transport to a mail exchanger for example.com. The slow transport could be configured to run at most one delivery process at a time:
When no transport is specified, Postfix uses the transport that matches the address domain class (see DESCRIPTION above). The following sends all mail for example.com and its subdomains to host gateway.example.com:
example.com :[gateway.example.com] .example.com :[gateway.example.com]
In the above example, the  suppress MX lookups. This prevents mail routing loops when your machine is primary MX host for example.com.
In the case of delivery via SMTP, one may specify hostname:service instead of just a host:
This directs mail for firstname.lastname@example.org to host bar.example port 2025. Instead of a numerical port a symbolic name may be used. Specify  around the hostname if MX lookups must be disabled.
The error mailer can be used to bounce mail:
.example.com error:mail for *.example.com is not deliverable
This section describes how the table lookups change when the table is given in the form of regular expressions. For a description of regular expression lookup table syntax, see regexp_table(5) or pcre_table(5).
Each pattern is a regular expression that is applied to the entire address being looked up. Thus, some.domain.hierarchy is not looked up via its parent domains, nor is user+foo@domain looked up as user@domain.
Patterns are applied in the order as specified in the table, until a pattern is found that matches the search string.
The trivial-rewrite(8) server disallows regular expression substitution of $1 etc. in regular expression lookup tables, because that could open a security hole (Postfix version 2.3 and later).
This section describes how the table lookups change when lookups are directed to a TCP-based server. For a description of the TCP client/server lookup protocol, see tcp_table(5). This feature is not available up to and including Postfix version 2.4.
The following main.cf parameters are especially relevant. The text below provides only a parameter summary. See postconf(5) for more details including examples.
trivial-rewrite(8), rewrite and resolve addresses master(5), master.cf file format postconf(5), configuration parameters postmap(1), Postfix lookup table manager
Use "postconf readme_directory" or "postconf html_directory" to locate this information.
ADDRESS_REWRITING_README, address rewriting guide DATABASE_README, Postfix lookup table overview FILTER_README, external content filter
The Secure Mailer license must be distributed with this software.
Wietse Venema IBM T.J. Watson Research P.O. Box 704 Yorktown Heights, NY 10598, USA Wietse Venema Google, Inc. 111 8th Avenue New York, NY 10011, USA