1. First of all login into your gmail account in which you want to cancel the mail.
2. Now at the right top corner you will see setting option. just click on it.
3. Now in the drop down list select settings.
4. Now under the setting tab you will see the section Labs just click on it.
5. Now scroll down and enable the Undo Send button there. As this will enable undo send feature of your account.
7. Now gmail will give you 10 seconds to undo every email when it is sent, but you can change this time in settings->general->undo send->cancellation period and set it to maximum of seconds.
8. Thats it everytime you send the mail it will dislay 10-30 seconds to undo the sent email.
So above is method for How To Cancel A Sent Email in Gmail. By this you can easily avoid sending some mails that you actually don’t what to send but you send them accidenlty at that case this method will be very helpful.