> if a NUL appears in the header somewhere all bets are off.
I think it would be fascinating to understand how that happened. Depending on how the parse tree is done, it could be marginally bad, or catastrophic.
I really would be amazed if this is seen in the wild. But its a big network: maybe its out there?
The IETF "modern SMTP" stuff John Klensin is working on (with others) might want to talk to that: a lot of the ICANN UA stuff is a push for UTF-8 clean across the board.
-G