There is one setting, underdocumented, which makes a big difference these days. spamc has a default ceiling of 10K, messages larger than which it passes uncheck

There is one setting, underdocumented, which makes a big difference these days. spamc has a default ceiling of 10K, messages larger than which it passes unchecked. Spammers have started routinely including images just over that threshold to defeat default installs. Bump that up a bit, and your accuracy will go way up.