And nobody has yet explained how the recipient of a classified email could have prevented it from being sent.
When you have a clearance, you (at least contractors do, I suppose government types might be different) have to undergo "training" on how to handle classified information. One of the things that you're taught is how to deal with receiving classified material on an unapproved computer.
Bottom line (for contractors at least) is that basically you immediately disconnect from the Internet, immediately stop using the computer (but do not close any running programs or turn it off), and immediately contact security who will start making arrangements to deal with it. Failure to do so is essentially a crime. (Or at least I sure thought it was. Apparently it isn't, if you're a Clinton.)
Receiving classified information isn't itself a crime, but receiving it and then not doing anything about it is. Or, at least, that's what I've been taught every time I'm forced to retake the Security Clearance Refresher Training.