DNS Blacklist Lookups
If you define the dnsbl feature (FEATURE(dnsbl) on page 261) or the enhdnsbl feature (FEATURE(enhdnsbl) on page 263) in your mc
configuration file, you will cause
sendmail to look up the IP
address of each connecting site at the blackhole
server you specify. If a lookup is successful and
returns a match, the connection is rejected, or as
of V8.14, discarded or quarantined. If a lookup is
successful and returns no match, the connection is
accepted. If the lookup fails, the connection is
either deferred or accepted, depending on the nature
of the failure.
Lookups are performed using the host database type
(dns on page 905). Each lookup
attempts to find A (address) records that correspond
to the address looked up. Note that this is
different from the usual way in which addresses are
looked up. Normally, addresses are reverse-looked-up
to find hostnames. But for blackhole purposes,
addresses are forward-looked-up, as though they are
hostnames.
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