Sometimes after sending an e-mail, you immediately realize it was a mistake. You slap your forehead and pray the recipient confuses it for spam or just thinks you're off your meds. Google understands your pain, and has introduced an Undo Send feature to Gmail.
Turn Undo Send on in Gmail Labs under the Settings panel of your Google account. Undo Send is hidden somewhere near the bottom of the page. Once the feature is enabled, an Undo option appears after you've sent a message. You have five seconds to click it. Hit it in enough time and you're good to go. Miss the mark and your acid-laced diatribe about your boss will see the light of day.
Undo Send functions a lot like Gmail's Mail Goggles feature that was seemingly made for the purpose of stopping intoxicated lonelyhearts from drenching exes with professions of love. The difference is that with Undo Send, you have to be super quick, and it's questionable whether someone accustomed to sending mistake messages would be fast enough to stop it.
I tested Undo Send by composing foul-mouthed messages to myself and then trying to zap them in time. Messages I sent from my Google account to my Google account appeared instantly and didn't offer me a chance to delete. But if you send them to an outside address, they will never arrive at their intended destination. To ensure my test was failsafe, I sent a bunch of really awful e-mails to friends and family and zapped those, too. It was quite cathartic.
Undo Send exploits Gmail's relative slowness. Slower e-mail clients -- are you listening, Outlook? -- should take note from today's news from Google Labs and replace clunky "recall message" options with an undo button. Gmail's Undo Send works, as long as you're quick with your regrets.