Most of the times after we have dropped a mail we think of making a change in the mail which has already
been sent making task impossible. But thanks to Gmail for providing their user with the option of enabling undo mail functionality which can be controlled by user. To make use of this functionality just login in Gmail navigate to the Settings menu while logged into your Gmail account via the web (Not for mobile).
The Settings menu is found by tapping on the gear icon in the upper right corner of the screen and then selecting Settings from the drop down menu. Within the Settings menu navigate select the General tab and scroll down until you see the Undo Send subsection. Here to make use of Gmail undo functionality do the following things :
1) Check Enable Undo Send
2) Set the amount of time you want Gmail to wait before sending the mail
Once this is done when next time you send a mail Gmail will display you with undo button for the mail for the specified time. But there is a constraint with it , it is only displayed till the time you remain on the page once you move away from the page it disappears even though you have time left for undo. This feature is only available till the time you are on the same page.