It doesn't send the email until after 0 - 30 seconds, so you have a chance to undo it :)
So, if I were to close the app at second 17, the email wouldn’t get sent?
That is correct.
However, would you think again if a little pop-up reminder told you just before you exited the app?
To be fair though, I keep my email tab opened all day. I never close it!