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 ls(1), touch(1),
and rm(1).

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.