Should email providers add "UNSEND"?
qhoxie: While it's not impossible, email just does not work that way.  Email has no sense of state on the other end of a message.  The extent of the feedback it gets is a bounce that may or may not have a descriptive error attached to it.  Some systems could implement a recall option (Exchange may allow this, but I'm far from an expert) if the accounts are housed in the same system, but it is rarely done as it is against the stateless aspect.  Email as a protocol(s) is very simplistic, which is why spam and spoofing are so prevalent.  There is little room to have complex extensions on top of the existing system.If, say, gmail implemented this, it would likely require an ACL from the receiving user that specified senders that were allowed to recall.