On the Developer Tutorials blog, there's a recent example of how to validate a common task of many signup forms - if the email sent is actually received.
For any web developer that's ever had to build a signup routine, email is the necessary evil that takes pride of place among hated activities. Sure, a simple call to the language's mail library will send a message through, but the moment the boss wants a HTML email, or users need attachments, everything starts to get tricky.
They try to solve the mail return issue (it tells if the message has gone to the queue, not been delivered) with a little testing and Fakemail. The software looks for emails based on the configuration given and pulls them out to a local directory. Your script can then look here and check the validity of the message before its sent.