"You Sent It, But Did They Read It?" is the mantra of DidTheyReadIt.com, who promise to be able to tell you when, where and for how long a recipient is reading your email. They don't actually mention how they do this, but it isn't that difficult to figure out. Lot's of spammers (and crackers) use what are called "web bugs" as a way of tracking by whom and when a bit of email is read. Its a tool well known to the sleazy.
[ The Virtual Quill ]
Taking a cue from Ximian (at least that's where I saw it first) Microsoft now (by default) doesn't download the pictures in your email (as of Outlook 2003 and Outlook Express with WinXP SP2) and thus stops this method from working. Hopefully all other clients will follow suit, and this horrific practice will stop. (Even if my RSS feed can support the same scheme).
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Disclaimer The opinions expressed herein are my own personal opinions.