sendmail, 4th Edition

FEATURE(mtamark)—V8.13 and Later, Experimental

One way to reduce spam email is to set up a mechanism for marking each MTA as an MTA. To illustrate, consider a spam email received from a host with the IP address 192.168.123.45, that claims to be a legitimate MTA. Currently, sendmail can only look up that address using various open relay sites to see whether the IP address corresponds to an open relay, and to reject the message if it does. Under the MTA mark proposal,^[139] sendmail can look up a special TXT record associated with that address to see whether that IP address is marked as that of an MTA. You may emulate this lookup using dig(1) like this:

% dig txt _perm._smtp._srv.45.123.168.192.in-addr.arpa

Here, the _perm._smtp._srv is a literal defined by the MTA mark proposal. The 45.123.168.192 is the original IP address reversed, and the in-addr.arpa is the special domain used to treat IP addresses like domain names.

This lookup can return one of two possible TXT records. A “1” means that this IP address is that of an MTA. A “0” (or any other character) means that this IP address is not that of an MTA. Mail from an unmarked MTA may, under this proposal, be rejected.

Once this proposal is in place, spam sites will no longer be able to send spam email via hijacked PCs, via hired PCs, or via worms implanted in PCs. When spam email does arrive, you will be certain that it is from a marked MTA and only from a marked MTA. Then, by blocking email from that IP address, you will be able to ...