@regression, pyro is talking about downloading images that are attached to the message. That's OK. The OP wants to automatically download messages on remote servers (embedded in the message source), which are often used by spammers for tracking live email addresses. Can't be done
AFAIK, and personally despite your undying faith in your spam filters, I would highly recommend against pursuing it. It's annoying, but all it takes is one to start the avalanche.