Yes, you can cancel email sending (even after pressing the ‘Send’ button).
Once you have sent an email, Gmail displays a message with an "Undo" button at the bottom left of the page.
Click on the button cancels the sending, returning you to editing. The default time for the button to display is 5 seconds. Than means that in 5 seconds the email will be sent and you will not be able to cancel it.
How to change the delay before sending
General tab, scroll down to
Undo Send and set the desired delay time.
Save Changes button (at the bottom of the settings page).
No matter what delay you set, take your time to send your email. We recommend that you read the email carefully before sending it. This will prevent unwanted dispatches and reduce the number of times the "Undo" button is used.