> before it actually leaves your computer Well, it leaves your computer as soon as you type on the keyboard; that's how it saves drafts. The Undo feature works

> before it actually leaves your computer Well, it leaves your computer as soon as you type on the keyboard; that's how it saves drafts. The Undo feature works by telling Gmail servers to send the email in X seconds. That timeout gets canceled if you hit Undo. Or at least I assume that's how it works. I'm sure it doesn't wait to send data to the Gmail server until timeout is over. If that were the case, your email would never be sent if you closed the tab during the timeout.