Why does email need an envelope, and what does the "envelope" mean?
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.
Answer
The recipient address in the SMTP RCPT TO: command is what mail transports use to determine the actual recipient of an email. The addresses in the To: and Cc: headers are there for mail readers to display to users who the recipients are, but they're not actually used by mail transports.
Most mail clients set the same addresses in SMTP RCPT TO: and MAIL FROM: commands that they insert in To:, Cc:, and From: headers, so the "envelope" addresses will be the same as the addresses in the headers.
Envelope and header addresses will usually agree for most legitimate mail. Notable exceptions are Bcc: addresses and mailing lists.
Spammers often forge header addresses to try to avoid spam filters.