
Step 1: Click on the icon "gear" in the upper right corner of your Gmail window, and select "Settings" menu.
Step 2: Select the "Lab" from the Settings menu.
Step 3: Scroll down, until you find the "Undo Send" and click on "Enable".
Step 4: Click: Settings> General> Undo Send, and select the duration of cancellation up to 30 seconds.
Step 5: Now after your email sent, there will be the option to undo!
Click here for detail picture
Good luck...
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