I run my own mail server on OS X Server. For the most part I have my SpamAssassin settings tweaked pretty well. Unfortunately, there is some spam that seems to have malformed or incomplete MIME boundaries and these messages don’t seem to be getting passed off from Amavisd to SpamAssassin. As such, they seem to be getting through to the recipient with a null value for the spam score.
After a little Googling I found Amavisd critical settings and I think the $final_bad_header_destiny is the key. The default setting is set to D_PASS. I’ve changed mine to D_REJECT now I’ll see what the logs say and see how much non-spam gets caught. Obviously, if enough good stuff doesn’t get through then I’ll have to think of something else.
But the obvoious question remains, Why doesn’t SpamAssassin score these messages?