You could in theory parse all HTML parts of messages, and identify every off-site link. Having done that, you could make a request for each link, preferably through a site-wide web-cache (so that the user's content is pre-cached if it ever gets to them), and then decide what to do based on what comes back.
Of course, if you do this, simply hitting the server triggers the "Aha, someone read this email" nature of the web bug, defeating the purpose of the filter.
Kelson Vibber
SpeedGate Communications <www.speed.net>
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