Neither the dialog nor the preview solves the problem that an undo button solves. If a dialog or preview comes up for every message you send, you just develop

Neither the dialog nor the preview solves the problem that an undo button solves. If a dialog or preview comes up for every message you send, you just develop a new habit for sending mail (press the send button, then press the send button again on the dialog / preview). If anything, it just creates more of a hassle for the user. The undo button is for that 'Oh crap!' moment, when your auto-pilot lead you astray.