Linux mail server: How to fight image spam

This is getting out of control these days.

There is an interesting article at LWN about Fighting image spam.

From the article:
A number of spammers have been evading filters like SpamAssassin (SA) recently by encoding their messages as images. SA already has a set of rules that are meant to combat image spam, but the more recent messages (typically for stock scams or pharmacy products) have been crafted to avoid them. This would indicate, once again, that spammers are using SA to pre-test their messages and are modifying them to get through. SA developers, however, are up to the challenge and two specific countermeasures have been released.