It doesn't have to be 5 minutes. It could be 15 seconds. I've used that feature (in Fastmail), and find it very valuable.

I'm confused. Doesn't gmail offer an 'undo' for send by default ? At least for the last 5 years ? It's in General settings "Undo Send" and can be set up to 30 seconds ?

And before that it was under the Labs experimental features, I think I enabled it in the late 2000s.