[pjmlp]

No you cannot, it is not part of the email protocol.

Once it is sent, it is gone.

I always have fun with those "Please delete messages" from Outlook sent to my email client.

[BrandonY]

Gmail does not literally unsend SMTP messages. To give you time to undo, Gmail delays sending the message for a few seconds. So if you don't select "Undo" within the time limit, your message will be sent.

How to use: https://support.google.com/mail/answer/1284885?hl=en

[thrownaway2424]

Note these instructions are for how to enable this on Gmail, the original. Inbox does this by default.

[thrownaway2424]

Yes, that was a nice strong statement but you are wrong, and the other guy is right. When you send a message from Inbox, you can cancel it, for a few seconds. It doesn't bother actually sending it until the Undo button has disappeared.

[pjmlp]

The statement I was replying to never mentioned pressing undo button within a specified interval.