[pjerem]

> (that works by delaying the send a bit).

A delay which can be annoying in its own way. It’s rarely an issue to be fair but it means you may not be informed of errors if you leave the mail client too soon. Also there are (admittedly very rare) cases where you want the mail to be sent immediately but it is sent at an unpredictable time : those undo features rarely display a countdown.

[Retric]

True, on the other hand such delays help reinforce that email asynchronous rather than real time like chat programs.

[IshKebab]

Sending email is inherently asynchronous so the errors aren't an issue really.