I would agree with the envelope analogy if the message was not encoded in plain text. Say base64--an intermediate has to go through a slight process to read it, like opening an envelope.

Mostly, it's worse than a postcard. It'd be like if every letter carrier makes a copy of the postcard, then is supposed to discard theirs once the next person has their copy. There is zero guarantee that any of the several copies were actually discarded, and it's highly improbable that they were erased securely by any of the intermediates.

Knowing that the messages are transmitted in plain text across at least two mail servers (often more), and across several intermediate ISPs, it is ... unlikely... for an informed person to have an expectation of privacy in email.

Encryption solves all these problems, but we all know that's not in mainstream use. Not even signing!