Every GMail user needs to get this enabled. ASAP. So it can be used whenever needed.
I’m pretty careful about what I say in email anyhow. Never curse or write anything I wouldn’t say to my pastor. 
But reply to group has burned me a few times discussing projects. An unsend would be helpful.
Now if Outlook would just add this feature life would be good. 
Nice! I don’t use Gmail, but maybe this will catch on. Looks like they really make you dig for it.
I just setup the option on my work email. It works. I see the undo for a few seconds after an email is sent.
Find the message in your Sent Items folder, open it, and under Other Actions, you have a couple choices for recalling a message:
http://www.groovypost.com/howto/unsend-email-outlook-2013-2010-recall/
I wouldn’t count on it. If the recipient has already opened his mail, you won’t be able to recall it. Or, even if he shares a server with you, you might be able to recall it, but that’s of limited use if he has already read and/or printed it out and/or stored it somewhere else.
I had this happen to me recently, when the sheriff’s office emailed me a bunch of public accident records, then frantically tried to recall them because they had forgotten to redact the personal information. Too late; I already had them.
This has been available for ages. We’ve had it set up on our work email (a skinned Gmail account) for at least a year IIRC.
Quite useful.
Musicat - it’s not a “recall” function, it’s an “undo sending” function, so the recipient can’t have read it already. The email delays sending for 10 seconds or so after you hit “send”. You can hit “Undo” during that time and the email never gets sent at all. The downside is the slight delay in sending, but the upside is that you always seem to notice that glaring typo or missed attachment just moments after you hit “Send”. This feature rescues you.
Completely worthless feature, IMO. I have regrets about emails I sent years ago.
Well, that’s the thing. It only delays the actual send function for 10 seconds or so. It’s no different than if you sat back and thought “do I really want to send this?” which is something you should always do with email messages in the first place.
Especially after midnight on the weekend, for example. 
Gmail Labs used to have an option to set certain times of the day/week where you were asked to solve simple arithmetic problems before the email would send 
Yeah, a 10 second “un-send” feature really isn’t much of a help.
But wouldn’t it be great if ,when Google glasses hit the market, they include a brainscan feature that will let you delete info from the wearer’s brain.? Call it the “un-see” feature.
Yeah, I first set that up on my Gmail a little over a year ago.
My technique is to avoid putting the recipient’s email address in the To field until I’ve written everything out and double-checked it. That goes in last. So if I hit Send by accident, it won’t go anywhere. I can’t see how this feature would be better.