You can view when your message was opened and if it was tampered with
along the way, although I am not sure I trust their system to tell me
the complete truth about the latter. The others offer some tracking
features as well.
Numerous other products out there claim to help you with securing your
email. PrivacyX.com, for example, provides an anonymous certificate for
encrypting your messages if you can figure out how to use it with your
email program. Products like Interosa, Sigaba and Disappearing also can
be used to secure your messages.
In short, the whole lot is just trouble. For the time being, I am still
in the dinosaur age of unencrypted email. Maybe if I have a few spare
hours some day, I will try to get those certs working with Outlook so
at least I can sign my messages. But I won't bet on it happening
anytime soon. That doesn't mean that I won't still feel guilty about it.