Have you been in the situation where you sent an email and then immediately went (in cinematographic drama slow motion) "Nooooooooo...! I did not mean to send that!"? I'm sure you have. I have.
If you use Gmail, there's a wonderful setting, Enable Undo Send. Set it to ten or twenty seconds, which will leave the message in the outbox instead of delivering it rightaway. Ten-ish seconds should be enough for you to get the oh-no -reaction and press Z to effectively unsend your message.
There's a similar trick for Microsoft Outlook. Create a Rule. Start with a blank rule applied when messages are sent (last on the list). Check no conditions unless you really want to tweak things. Defer the delivery by one minute (again, the last setting). No exceptions necessary (unless, again, you really want to tweak things). Give your rule a nifty name ("Delay sending by one minute") and that's it!
Outlook has a feature to "Recall a message". What it in all practice does is send a second message to the original recipient telling that you wanted to recall the message. That if anything would make the recipient want to go wow, what's he written now?. The funny thing is that your Outlook will just look smug and say that it's recalled the message (or at least attempted to). Oh yeah. Or you can buy an Exchange module to have the server delay the message deliveries, but all in all this is the cheaper option :)