A rule to match any domain literal

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If the MTA tries to rewrite an address whose domain is of the form [n.m.p.q] 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.


See also: