Thank you for your feedback. I see how this can be confusing. The first chapter is explicitly about user-facing concepts, though. I explain message envelopes here: https://explained-from-first-principles.com/email/#message-v...

Right afterwards, you find an example of how and why the envelope diverges from the message header fields, followed by an in detail treatment of who removes the Bcc header field, and how Gmail recovers the Bcc recipients.