Instead, I'm suggesting applying S/MIME or PGP to the *server-to-server*
communications. In effect, the first hop MTA server e-mails to the last
hop MTA server the end user's e-mail, both headers and message body, as a
signed e-mail attachment.
What's your transition model? It'll be a long time before we can assume
that recipient MTAs understand MIME and how to unwrap encapsulated
messages.
Regards,
John Levine, johnl(_at_)iecc(_dot_)com, Primary Perpetrator of "The Internet
for Dummies",
Information Superhighwayman wanna-be, http://iecc.com/johnl, Mayor
"I dropped the toothpaste", said Tom, crestfallenly.