While I was checking out a new email client I wanted to see how it behaved in handling mail. I came across this fantastic site that allows you to run a test on your email client and see how it is REALLY handling your email.
From the site:
Some email clients perform operations when reading an email which give away information about the reader, to the sender of the message. If you enter your email address below, this application will send you a specially crafted email which uses a variety of techniques, to attempt to send information back to this server when read. It will then display the results for you.