Peter,
I agree with your conclusions, but not your
rationale. One of the nice things aobut PEM
is that it will work over a _virtual_ network,
including sneaker-net.
The reason why we don't use the mail headers
for ANY purpose within PEM is that (1) they are
not trusted (as they may be modified en route),
(2) they may be discarded entirely by some
applications, notably for archiving of the message
itself.
Granted, there are certain aspects of PEM which
were designed to make coexistance with SMTP
relatively painless, such as the choice of the
character set and the demarkation indicators.
But a mailer envirnment is not and should not be
a requirement for PEM to function. It will work
over SMTP, or X.400, or Microsoft mail, or Lotus
Notes; or you can write the information on a floppy
disk and bury it under a rock (special case of FTP).
Bob