Short answer: Not possible Long answer: Yes, but it would take a lot of work. You could hack into your recipient's e-mail and delete it. You could bring down the server where the recipient's e-mail resides. You could devote the rest of your life to discovering a method of time travel, then travel back in time to prevent yourself from sending it. Of course, you would probably be risking the integrity of the space-time, and you'd end up with a paradoxical situation (since you didn't send the e-mail, you wouldn't have devoted your life to discovering time-travel, thus you wouldn't have gone back to prevent it, thus restarting the cycle.) Oh yeah, this is assuming that time travel is possible in the first place. *Edit* Sigh, took too long to type in a response...