osxdaily reports: [edited]
If you’ve ever sent an email that you immediately realised wasn’t complete, contained errors, or, even worse, was to the wrong person, you know that feeling of dread. For web-browser Gmail users, an optional setting offers a layer of forgiveness to those situations, providing the ability to recall a sent email.
Go to gmail.com and log in to your Google account as usual.
Click on the gear icon/settings button in the upper right corner of the Gmail inbox window, then choose the “Settings” option from the pulldown menu.
Select the “Labs” tab. Locate “Undo Send” by ‘Yuzo F’, check the “Enable” radio box and click 'Save Changes' at the bottom of the window.
Next, go to the “General” tab within Gmail Settings, locate the “Undo Send” option and adjust your recall email/undo send cancellation period. Again scroll to the bottom and select “Save Changes”.
To use the feature, send any email, and you’ll find an “Undo” option available next to the “Your message was sent” box that shows up in yellow at the top of the screen.
When you click “Undo” the email becomes unsent, returns and becomes a draft, allowing you to amend and resend the message.