We recently implemented spamassassin on our server to mitigate spam volumes, and it does it quite well... except for one thing. For reasons I can't determine, it will suddenly start chewing through RAM and cpu usage until either spamd dies or the server becomes unresponsive.
What version are you on?
3.0.1 (Just released) fixes a few memory-hogging corner cases in SA. Especially if you use ok_languages or bayes.
