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. }