There is new software that allows one to retrieve an email after sending. Don't know the details, but I guess it may zap it enroute or snatch it out of the recipient's in-box if it's not yet been read.
We had that on the internal system at my employer in the early 90's. If the message hadn't been read, the content would be removed, but the recipient would have a notice that said, "Message recalled by sender" or something like that.
Once an email makes it to the POP server of the intended target, aint nothing you can do.
I think you can do it in an inhouse email server where you send something to your boss or something since all the email transactions are done in house but once it leave the in house server to the internet, asta la vista, email.