However, in my previous comment, I didn't express myself precisely enough. I didn't mean "block" or "let through" rather "execute test and set specified score if the test turns true" so that the final decision what to do with the mail could be affected by the other tests. Very often you also want to do something else than the simple block or pass, such as repackage and mark, give the user a hint but let him decide. AFAIK this you cannot do in an SMTP server. You also want to gather together all spam related work in one place.
That makes sense, and is a good application for SA.
In any event, adding custom rules is pretty easy.. And if you're using SA 3.0 this is very easy since SA pre-parses some of the Received: headers for you into a fake header you can write header rules for....
header L_RELAY1 X-Spam-Relays-Untrusted =~ /\[1\.1\.1\.1\]/
score L_RELAY1 -1.0
describe L_RELAY1 Address 1.1.1.1 was a relay of this message.
