[mrduncan]

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).

[pilif]

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.