All of the DNS blacklists we used a couple years ago are defunct. There are, however, a slew of new DNS-based blacklists that can help protect your users from spam. Understand that with any blacklist there is a chance that legitimate email will be blocked. Balancing this against the cost and annoyance of spam is something you should discuss with your users. We recently put the following blacklists in place, and cut out virtually all spam to our users:
FEATURE(`dnsbl',`relays.ordb.org', `Rejected - see http://ordb.org/')dnl FEATURE(`dnsbl',`bl.spamcop.net',`Rejected - see http://spamcop.net/')dnl FEATURE(`dnsbl',`sbl.spamhaus.org',`Rejected - see http://www.spamhaus.org/')dnl
Just put this in your sendmail.mc file. For other MTAs, check out Spamcop, Spamhaus, and ORDB.
[ed. note: Orbz is shutting down as of 3/20/02. If you are using Orbz, remove it, because it will slow down your mail if you don’t. For more DNSBL-style spam deterrents, check out This site]