HERE'S HOW YOU CAN UNSEND AN EMAIL IN  GMAIL

STEP 1 : First of all, click on the settings icon at the top right on the gmail page.

STEP 2 : Now click on the general tab.

STEP 3: You will see the optinon of undo send

STEP 4 : Now you have to set the time. However,  the default time is 15 seconds

STEP 5 : However, you will get the options of up to 5,10,20 and 30 seconds.Hence you can schedule it for 30 seconds.

STEP 6 : Now save the scheduled email,and your settings will be changed

STEP 7 : Now whenever you send the emai, you will see an option on the left side of the screen in which the first message sent will be written. After this, undo and view message will be reported.

STEP 8 : Now you can click on the undo option if you have accidentally sent the email to someone.This will return the email immediately