The Most Effective Block

This may be a bit premature, but I've noticed lately that neither spamassassin nor spambayes are flagging a large number of messages as spam ...yet spam isn't appearing in my inbox. Statistical reports on my mail server indicated that on the order of 10% of the messages processed were rejected at the door². The most effective blocks appear to be (in descending order of rejects) :

• "freemail" sender/client sanity check
• Syntactically invalid IP address literal in HELO command
• Not fully-qualified hostname in HELO command
• My own hostname presented in HELO command
• Domain of sender address doesn't exist³
• My own IP address presented in HELO command

Naturally these checks must all be done at the mail server. The freemail access check appears, by far, to be the most effective. One explanation for it can be found in http://jimsun.linxnet.com/misc/postfix-anti-UCE.txt.