I don't see a problem with it if the translation is 1-to-1 (and that's a pretty big if). Disregarding federal law for the moment, what is the harm in the mailman reprinting your letters with nicer font on nicer paper? It seems to me that if the mailman wants to do that, as long as they don't lose any of the information of the original letter I would benefit from having an easier to read letter.

But if it becomes normal for a mailman to do this, there is now an expected man-in-the-middle. Sure the mailman might be benign, but what about if there is another man-in-the-middle (less benign), now that I expect my communication to be tampered with I won't notice anything suspicious. [Note: Zed's proposed solution is to forward on the original mail as well so it can be verified]

I'll confess now to definitely not being an expert on mail systems but what is to stop this tampering from happening now by a man-in-the-middle? I don't really see any new avenue of attack that doesn't already exist with current systems (encrypted email excluded).

no one is to stop tampering from happening.

but the SMTP standard and the whole culture around internet mail mandates that the messages are not changed in transit (with the exception of that received header and the stuff around 8bitmime).

UTF-8-Encoding mails just because it's "cleaner" doesn't feel like it's the right thing to do, especially when you consider there to be old systems around that can't handle UTF-8 encoded messages.

Also, standards are there to be adhered to - like HTML and all the others.