I could actually see myself using revoke mail, but more on desktop than iPhone. I remember back in the pre-wifi era, I would sometimes realize I forgot something in an email and quickly yank you my ethernet cable. I've tried doing the modern version of this (turn of wifi), but I have never been able to get to it in time. Perhaps I should make a function key that turns off wifi (or force-quits Mail?).

Plug the router out?

It's very far away! I suppose the fastest thing I could do is sleep my computer, and then turn off the router before waking it back up. But I don't know if a sleep command will interrupt the process of sending a mail message. I suspect it would if a large upload was involved, but I have my doubts about a smaller message.

Maybe use a hotkey to tell a smart plug to power off? You wouldn't be able to use that to turn it back on with your router being disconnected but that can be done by hand.

The revoke mail feature has been a feature in gmail for 10+ years (though it probably was a “labs” feature at first). “Undo send”. And there, yeah, it’s just a configurable delay - I use 15s. I’ve used it (and been thankful for it) many times