undo send is dumb and the worst name/implementation.
i'd either add a dialog (with default selected button being Cancel) or replace the Send button with a Preview button, which would show the message the same way you read a message, and there you would have the Send button.
Undo send is great. It lets you undo the one time in a hundred that you make a mistake. Your solution would require that everybody click through an additional dialog on every single email sent.
Letting people recover from uncommon mistakes is better than ham-fisted and annoying mechanisms that attempt to prevent the mistake.
"undo" email does nothing else than create a false sense of security.
It probably works for the impatient that is hitting ctrl+enter (or whatever shortcut) too soon by muscle memory. but then it should be called delayed send to not give a false mental model to the user.
the current model is preventing the real mistakes for probably 1/10 of the cases it's needed. and enticing less care and probably bumping the rate in the end.
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.
i'd either add a dialog (with default selected button being Cancel) or replace the Send button with a Preview button, which would show the message the same way you read a message, and there you would have the Send button.