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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.

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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.

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  • $\begingroup$ Usenet has the "Supersedes:" header, which may or may not also be handled by some email clients. However, this does not "unsend" the message, rather it sends a second message asking the email client to replace the original message with this one (and there is a specific protocol for deleting as well). However, all Usenet clients I have used, clearly mark such messages and give access to both the original message and the raw version of the Supersedes message for inspection. Essentially, this makes Usenet an immutable graph database. $\endgroup$ – Jörg W Mittag Mar 8 '20 at 9:04
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Either you catch the mail before it goes out (locally, or wherever your mail relay is), or you convince the recipient's mail spool to delete the already arrived message. If the recipient got the mail before the "delete" message arrives, though luck.

Personally, I've never seen anything offering this functionality. If the mail is handled purely locally (or origin/destination use the sane software offering this), it could be done. No standard way, that's for sure.

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    $\begingroup$ Gmail allows you to unsend an email. The way this works is simply that the email isn't sent when you hit "Send", it is sent when the timeout for the "Do you want to unsend?" message expires. $\endgroup$ – Jörg W Mittag Mar 8 '20 at 9:02

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