make marshal respect /mail/box/$user/headers when forwarding or
replying to a message from nedmail (I.E. using -n option).

MS Exchange insists I use the same username in the smtp
envelope as the rcf822 header. I enforce this in /mail/lib/remotemail
(smtp) and /mail/box/$user/headers (rcf822) so both must be used
when forwarding as well as sending mail.

-Steve