http://www.snpx.com/cgi-bin/securitynews.cgi?database=JanDD&id=74

If you run Sophos virus scanning on your mail server, there is a hole you might want to look at and 
plug the holes.  Link at bottom of article.

If you run a qmail server you have the other half of the problem.  If configured in a certain way, 
qmail will include the original e-mail in a bounced e-mail and not include MIME boundary 
definitions and so slip through.  So please disable this on your server.

Brian Kelsay