Gmail Undo sent email
As we know human makes mistakes many times. We all face such issues to send email accidently incomplete or different recipient. Therefor google team thought and observe this intendent and integrate a new feature for Gmail users.
Gmail undo options will work
It will basically allow you to recall a sent email within 30 seconds. If you have sent a message in hurry and thought, there’s some error that needs to be fixed. This this situation this feature of Gmail will gives you maximum 30 seconds to recall. After that it is not possible to undo your sent emails.
How to enable Gmail undo Feature
The feature is turned off by default for all. If you assume that such incidents you do face. Just enable this option for future mistakes.
Go to Settings gear icon right of the Gmail screen –> General Tab

Gmail undo in Email Clients
This option will not work for Desktop or mobile