Check out this neat thing: Email Privacy Tester
Smart web developer and systems admin Mike Cardwell put together something neat that I think you'll want to check out: Email Privacy Tester. Plug in your email address, and his system will send you an email message. That message will contain a whole bunch of different types of active content in various different wrappers and encodings, to see which of them are triggered by your MUA (mail user agent; aka email client). I did a quick test to a Gmail account and I already see one thing that I don't like, something Gmail executes automatically without asking me.