Dan Kohn writes:
a UTF8HEADERS ESMTP extension
No! 8-bit support is not an option. 8-bit support is not something to be
negotiated. 8-bit support is _required_. Do you want implementors in
twenty years to be dealing with 7-bit MTAs?
The challenge is that any mail relay implementing this
extension MUST be able to downconvert headers to 7-bit RFC 2047 
compliant encoded words if the next hop does not support UTF8HEADERS.
Sorry to bother you with the facts, but the 8BITMIME requirement that
you're mindlessly imitating is flagrantly violated in practice by many
MTAs, including most versions of sendmail. (All versions from 6.57 in
1993 until 8.7 in 1995; at least a few years after that if the popular 8
flag is set; I haven't checked the most recent versions.)
Furthermore, converting an 8-bit email address to 7 bits would destroy
interoperability. This isn't just a Q-P display failure; this is mail
delivered to the wrong address. Reliably undoing the conversion means
upgrading every mail program on the Internet, rather than just the 7-bit
programs that are actually at fault.
---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago