What are the underlying mechanisms allowing us to unsend an email? I am currently studying Computer Networking, and I was reading about the different types of Application Layer Protocols. In the book I am reading (Computer Networking: A Top-down Approach) it is mentioned that: "Once the client
sends a message into its socket interface, the message is out of the client’s hands and is
“in the hands” of TCP."

For the sake of example, I thought of the option we have of cancelling the emission of an email? 
Is this functionality simply implemented via a time-window of seconds before emission or is their an actual procedure I do not know of ?

Do please correct my assumptions as I am afraid I might be off topic on several points

Thank you in advance.
The e-mail system has no way of manipulating already sent e-mails, no.

The only services I've seen which allow you to "unsend" an e-mail simply delay the e-mail being sent for a user-determined amount of time and then allow you to cancel the send before it happens. 

Alternatively, I've seen some services allow you to "unsend" e-mails if the recipient is using the same e-mail client as the sender, as both sides are controlled by the same service. Although, once again, this is handled by the service provider, not by the e-mail system itself.