I know about the Undo Send labs plugin and it works pretty well. But the thing is when I click view message to check how it looks like after it has already sent, then the undo feature disappears. So I'm wondering if there is any way to undo an email when I have already viewed the message?
Put simply, no. The Undo Send button isn't really calling the email back after being sent, because that would be impossible. In reality, Gmail is just waiting a few seconds before it actually sends the email, giving you a moment to think twice about it. Once the timer runs out, or you click away on something, the email is sent and you can't get it back.