Can you 'unsend' a sent e-mail that is already delivered?
you cant retract a sent mail but for future enable the setting for undo send ( click the gear icon in gmail > settings > general > undo send > enable > set delay time > save )
note it only gives you up to 30 seconds max IF you dont click on anything else for those 30 seconds because all it really does is DELAY sending
after you send an email you will see this : Your message has been sent. Undo View message
p.s even it was possible - that email could have been forwarded automatically or manually or printed or downloaded so .......there is no way to guarantee you retract the only copy
another undo option , but i have not tried this chrome extension :