Here is a good reference site for you about this problem, also known as <a href=”http://spamlinks.net/prevent-secure-backscatter.htm”>backscatter.</a>
And another good reference page can be found <a href=”http://www.spamresource.com/2007/02/backscatter-what-is-it-how-do-i-stop-it.html”>here</a>.
Backscatter, unfortunately, is a problem there is no instant magic solution for, but those sites should give you somewhere to start.
You may want to switch spam filters or revert spam definitions.
SpamBayes (free) written in Python is a great filter.