No DNS consistency checks in Debian spam filter?

[Mike Bird <mgb@yosemite.net>]

The single most powerful and most efficient spam filter test is to
verify DNS consistency.  Judging by the spam now swamping
these lists, Debian does not employ such a test.

In Postfix, including reject_unknown_client_hostname at an
appropriate spot in smtpd_client_restrictions does the trick.

Alternatively, us old PERL bashers iterate over all PTR records
for the IP address, and for each of those PTR records we iterate
over all the A records, and we only accept the connection if at
least one of those A records contains the connecting IP.

Unfortunately, once Debian has accepted this garbage and
forwarded it to a million victims, its much harder to block.

--Mike Bird