The "Undo Send" feature is very useful, but currently sending an email goes through two phases. First, when I click the "Send" button on the Compose screen, I get a "Sending message..." with a spinner. That takes a few seconds, and then you get "Message sent." with the Undo button.

I think the user experience would improve if the Undo button would be available during the "Sending message..." phase already. Admittedly, it's a small change and improvement, but the frustration whenever you accidentally clicked the Send button too early, to have to wait for the Undo button to appear, is a very annoying one.