Does SM have a "trick" address feature? (I'm not really sure what it's technically called).
But I saw the feature available for some other mail server product I think a long time ago.
It works like so:
An email address is posted on a page in such a way that only an email harvesting expedition should cause it to be used and therefore any emails that go to the address are taken and added to a black list of some sort. I don't remember if it's by IP or return path..