Undo Send for Gmail on the web
Previously a popular feature in Gmail Labs, and recently added to Inbox by Gmail, today we’re adding ‘Undo Send’ as a formal setting in Gmail on the web.
‘Undo Send’ allows people using Gmail to cancel a sent mail if they have second thoughts immediately after sending. The feature is turned off by default for those not currently using the Labs version, and can be enabled from the General tab in Gmail settings.
People currently using the Labs version of ‘Undo Send’ will have the setting turned on by default at launch.
Rapid release, with Scheduled release coming in two weeks
Reference by Google