A rule to match any domain literal

If the MTA tries to rewrite an address whose domain is of the form     and fails, it tries one extra rule (prior to trying , and then the  " " match-all rule). This extra rule is the IP literal match-all rule. The pattern is. The pattern never changes. This rule is only activated when more specific probes including all or part of the IP address have not matched. (See Handling of domain literals for a discussion of the order in which portions of an IP address are checked for a match.)

In particular, a rewrite rule using the  pattern and with    in the template (thus meaning that the rewrite rule applies only to envelope From addresses) is typically used in modern MTA configurations to perform a lookup of a pseudo-address constructed from the incoming source IP of SMTP connections against the    mapping table and then to "switch" the incoming SMTP connection to an "internal" channel such as , if appropriate.

See also:
 * A rule to match any address
 * Rewriting: domain literals
 * INTERNAL_IP mapping table
 * Typical TCPIP channels and servers
 * Rewrite rule patterns and tags