Milter-skem

Cache earlier sendmail’s verdicts for the relays

The skem utility is a sendmail milter, that checks and maintains a list of whitelisted, temporary banned, and permanently blacklisted IP-addresses. How you obtain the entries is up to you, but the included logwatcher module provides one possibility.

The list is stored in a directory, each entry being a file usually – zero sized or a symlink usually – a “broken” one. Such entries are stored efficiently within the directory itself and the directories are searched using the hash tables on modern file systems. At the same time, they can be listed, added, and removed with the simple ls1, touch1, and rm1.

This milter does not itself filter spam, instead it memorizes the verdicts issued by your other anti-spam defenses to reduce the system load and resource consumption, by temporarily rejecting the relays suspected of spamming banned and, optionally, by permanently rejecting the relays “convicted” of spamming blacklisted.

The idea is to stem the spam from real spam sources, while reducing the ill effects of false-positives to merely delaying, rather than rejecting future messages.