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They're MUA headers, not MTA headers. That's an important distinction; those headers are generated directly by the end-users' systems, rather than by the systems responsible for routing the mail around.

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Even that isn't a clear bright line. For example, the To: header also comes from the MUA, and it's crucial to the message's transit. (In fact, the To: header is very much analogous to the stuff written on the outside of an envelope.)

To: actually isn't used in routing. MTAs and MDAs route messages based on the target address that's communicated in the 'RCPT TO' command, ignoring whatever may be in the To/CC[/BCC] lines, should the MUA happen to generate those headers.

The set of metadata used in routing, aka the 'envelope', also contains a return path, which is (normally) used for things like submitting delivery failure reports.

No, the To: and From: fields you see in your MUA is the software equivalent of my saying over the phone, "Hey, this is Steve - is Dave there?" In my fairly educated opinion, of course.

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