If you use a email client that opens external links via html email then take note of this.
HTML email is a privacy risk because it's trivially easy for the sender to embed a unique web bug inside an HTML email message that gets first when the mail is read (or even previewed within the preview area in Outlook and Outlook Express). That web bug is a small invisible graphic with a unique URL on a web server of the senders choice. When the mail is read the web bug is downloaded and the web server it came from makes a note of the image downloaded. By making the name of the image unique for each mail sent out a curious emailer can tell whether you downloaded and read the mail, and when and which ISP you were connected to, ...
If you include this little piece of HTML in the email you send it'll fire a web bug on UseTheSource that will record the name of the recipient. (Just replace the word email with the email address of the person you were sending the message to.
The recipient will not see the image (it's a transparent 1x1 dot) but my web server will record the email address after the ? in my logs. You, or a malicious person, could do the same thing. And spammers do this all the time; that way they can reliably tell whether your email address is worth spamming!