"Nobody, from the lowliest spammer to the grand exulted CEO of a massive company, can remove or change the content of an email message they have sent to you."

Unless the message has embedded images linked from the web. Those images are part of the 'content of an email' as far as most people are concerned, but aren't part of the message envelope. So they can be destroyed or replaced by the sender whenever they want.

Do any mail clients aim to deal with this by keeping a permanent copy? (Including the 1x1 tracking pixel content?)