Summary: GMail added the ability to "undo" a send. I'll look at how it works, and why it's not what people expect.
I heard that Google added an "undo" to their send button. How can that work? I thought you said that once you send an email it's out of your control?
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(This is anticipatory - I expect people to be asking.)
I stand by that statement: GMail's "undo send" is not really an "undo" at all. It's certainly helpful, but it's not what you might think.
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The undo send feature, which you'll find in Settings -> Labs, is really more like a "delay before sending" feature. When enabled, it "holds on" to your email for 5 seconds before the email is really sent.

5 seconds.
That's all the time you have to rethink your send.
After that time has elapsed, there is no undo.
It works because GMail doesn't actually send your mail for 5 seconds. They're holding on to it before sending it, in case you want to change your mind.
This is actually very similar to a technique mentioned by Outlook users, where you can set up a rule to delay the send of a message by some amount of time. Change your mind within that time, and you can stop Outlook from sending your message.
But I do have to reiterate, once your email has actually been sent, there's no way to "undo" it; there's no way to get it back.
(OK, a caveat for the pedants: some corporate email systems - where everyone is on the same mail system such as Microsoft Exchange - do, sometimes, have the ability for a true "remove it from the recipient's inbox" undo between users on the same system. But this is not available for internet email or for the average user on the internet.)
So certainly, enable the feature - it's kinda cool, and has already saved several people from sending erroneous or incomplete messages. But realize what it is, and what it is not.
Think before you send. No matter what.
Related:
Can I stop or 'un-send' an email I sent by mistake? Once you hit that "send" button, there is no way to stop your email from being sent, even if it's to the wrong person.
How do I get a notification when anyone I send an email to forwards that email? Several urban legends would have you believe that when you forward an email it can somehow be tracked. It cannot. There's no way to know.
Article C3682 - March 21, 2009
it seems the Labs tab does not appear in all language versions of GMail. Why is that?
24-Mar-2009
From Gmail website :
Gmail labs aren't yet compatible with all of our supported browsers. Labs are available only in Internet Explorer 7.0+, Firefox 2.0+, Safari 3.0+, and Google Chrome. You won't see a 'Labs' tab in your Settings if you're not using the newer version of Gmail on the browsers listed above. Please note that if you're using Internet Explorer 6.0, you will not have Gmail Labs.
Posted by: Michael at March 24, 2009 12:59 PMAOL allows you to "unsend" mail to other AOL members (if you use their mail program of course - I use Thunderbird). However, it can only be done with OTHER AOL members (I assume it is held in some AOL server somewhere waiting to be opened). It would be nice if the entire Internet used a system that EVERYBODY to use (other than AOL of course), then this feature could be implemented throughout.
Posted by: Carl R. Goodwin at March 24, 2009 1:53 PMrecently i'm having attachment problems with Gmail. I'm not able to attach any files. Is it only me or any of u people experiencing such problems
Posted by: Anand at March 27, 2009 8:28 PM@Anand
See if you have the flash uploader enabled. If you do, you'll need to have the flash plugin installed in your browser, which may be your problem.
Posted by: will at March 30, 2009 7:45 AM