postmap -q "string" /etc/postfix/transport
postmap -q – /etc/postfix/transport <inputfile
The optional transport(5) table specifies a mapping from email
addresses to message delivery transports and next-hop destinations.
Message delivery transports such as local or smtp
are defined in the master.cf file, and next-hop
destinations are typically hosts or domain names. The
table is searched by the trivial-rewrite(8) daemon.
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
Note 1: the null recipient address is looked up as
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
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
In order to send mail for example.com and its subdomains
via the uucp transport to the UUCP host named 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:
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)
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 [email protected] looked up as [email protected].
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
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA