Why does email need an envelope, and what does the "envelope" mean?

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What is the exact difference between the envelope and the email in smtp?

Why does the protocol need the envelope? In snail mail, the letter needs not contain addresses and is not visible to the postman (at least that's what you want to believe), so all the routing is made entirely based on the envelope and that is clear to me.

However in e-mail, the letter itself is parseable and has headers such as From: and To:. Why is that not enough to route email? Why do we need an envelope?

And what is the meaning of a divergence between the envelope and the email itself? Does it necessarily mean that someone is trying to game the system, or is it, under certain circumstances (what circumstances?), legal to have a difference between them.

Your clarifications are more than welcome.

Puzzled email receiver.

rfc822 smtp security email
4 Answers

An email message is very similar to a physical letter that you would send in the mail. There is an envelope, with To/From information, and there is the actual letter on the inside, with it's own To/From information. The envelope to/from information is the real information that is used for message delivery, for both email servers and post offices.

When an envelope comes into a post office, they inspect the To address on the envelope, and send it to the correct destination. The post office workers have no knowledge of the letter inside the envelope. The letting inside could have completely different To/From information than the envelope says. The Envelope could say the message is to Bob, but the letter inside may say it's for Alice. Or in real world: The envelope could say the message is to SomeCompanyName, and when secretary open envelope, letter inside say, it's is for Mr. Brown which work in SomeCompanyName.

Source: Email Envelope vs Email Header

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