Hi,
I wanted to share an idea:

I think that the current "UNDO send" in Gmail is almost completely useless. It would work much better if the text of your sent mail remains in the screen while having the chance to undo it. Maybe the background color could change, or the letters color turn gray, or something like that. That way you could "see" something that makes you change your mind about the mail you just tried to send.

Greetings from Ushuaia,

Heguido