wow Google keeps bringing a lot of really helpful features to GMail 
"Sometimes I regret sending a message the morning after. Other times I send a message and then immediately notice a mistake. I forget to attach a file or email the birthday girl that I can't make her surprise party. I can rush to close my browser or unplug the Internet — but Gmail almost always wins that race. (...) I could undo just about any other action in Gmail — why couldn't I undo send? (...) My theory (which others shared) was that even just five seconds would be enough time to catch most of those regrettable emails," says Michael Leggett from Google. I don't think that 5 seconds are enough to realize that sending the message was a mistake, but it's a good trade-off between functionality and the need to provide a reliable service. A simple improvement could be a configurable delay before sending messages.{ Thanks, Niranjan. }
via Google Operating System by Alex Chitu on 3/19/09
If you ever send a Gmail message too early or you change your mind after you press "Send", there's a feature that will help you. It's called "Undo Send" and you can find in the crowded space of experimental features from Gmail Labs.After enabling the feature, Gmail will show an "undo" link when you send a message. You have to react quickly because the link disappears in 5 seconds and there's no way to bring it back. If keyboard shortcuts are enabled in your Gmail account, a better option than clicking on "undo" is to press "z". When you undo sending a mail, Gmail saves it as a draft and you can continue editing the message or discard it."Sometimes I regret sending a message the morning after. Other times I send a message and then immediately notice a mistake. I forget to attach a file or email the birthday girl that I can't make her surprise party. I can rush to close my browser or unplug the Internet — but Gmail almost always wins that race. (...) I could undo just about any other action in Gmail — why couldn't I undo send? (...) My theory (which others shared) was that even just five seconds would be enough time to catch most of those regrettable emails," says Michael Leggett from Google. I don't think that 5 seconds are enough to realize that sending the message was a mistake, but it's a good trade-off between functionality and the need to provide a reliable service. A simple improvement could be a configurable delay before sending messages.{ Thanks, Niranjan. }