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
$E$R 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
INTERNAL_IP mapping table and then to "switch" the incoming SMTP connection to an "internal" channel such as
tcp_intranet, if appropriate.