yeah they should !
but thinking of practical implementations, they would be a nightmare to implement in the current way email is handled.
though gmail has a beta service just to prevent accidental sending of emails.. a very naive attempt but nonetheless http://gmailblog.blogspot.com/2008/1...you-later.html
Outlook permits you to recall and resend a message. Of course, it doesn't stop the recipient from reading the original, but it can be useful from time to time.
Thunderbird has a nice feature that warns you if it suspects you haven't attached an attachment.
Definition of the nonosecond. The time taken between hitting send and realising that you really shouldn't have.