I'd stop using Gmail, personally. To me, it feels like the receiver has a right to read an email he receives. It would feel... intrusive... for the app to decide to delete some of my data when I'm not looking (or rather when I haven't looked yet).

That said, the "panic button" undo feature seems like a great idea. But 5 seconds is way too short. (I'm envisioning my Dad using this feature in this case, not a techie.) 30 seconds seems perfect -- it's probably not a big deal that your message gets delayed by 30 seconds, and it gives you some 'think time' to remember a detail you forgot to add or fix.