Login to your Gmail account and click on the settings option on the top
Check for the “Labs” Tab in your settings
Scroll down, Near the bottom you will find an Undo send app for Gmail by Yuzo, select “Enable” option beside the app and the save your settings
Now you are done, By default the undo time is 10 seconds but if you want you can customize it by going to your general settings
Now whenever you will send an e-mail in the sent mail notification option ( appears just after sending in yellow background) you will see an Undo option which you can use only within the selected time.
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