Criptext

Encrypt, track, and recall your emails

Reviews
Discussion
Looks like this turns your email into an image that needs to get pulled from their server each time the email is opened (didn't know this but I guess email clients don't cache the email content). If you ever decide to recall the email or set an expiration date the recipient won't be able to access the content.
This could be so valuable. How many times did I wish I didn't hit send?! Will try it out and report back.
Criptext GMail Plugin works on: Chrome and Safari, Criptext Mail Plugin works on Outlook