We've had the ability to "unsend" emails in Gmail for about six years now, but the feature was somewhat hidden in the obscure Labs section of Gmail's settings. Google officially announced on Tuesday that "undo send" is an option in its Gmail service, and it's much easier to find in Gmail's settings.
Before digging around the settings to enable undo send, you need to reload Gmail, otherwise you won't find the feature.
Then click the gear icon on the top left of your inbox.
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You'll be taken to the general settings in the "General" tab. Look/scroll down to find the undo send feature, and click the box to enable it.
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Next time you send an email, you'll notice a subtle new "undo" option in the yellow box that appears at the top of the Gmail window after you send an email. If you realized you missed something or notice a typo in your email, click the "undo" link, and your email will reopen in the "compose email" box for you to edit.
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Unfortunately, there's no such option in the iOS or Android Gmail mobile apps where small mobile device keyboards means our emails are more prone to mistakes.