Exactly what in the process of adding a line (or two) at the beginning
of the data stream from the network to disk makes you believe MTAs know
about 2822 content?
MTAs will often add Message-ID, and Date headers because email
client don't. To do so an MTA must parse and understand the
basic headers of a message. ...
There's a chronic confusion in 2821/2822 mail between mail submission,
in which a MUA sends a new message, ant mail transfer, in which a
message goes from one MTA to another. In a more perfect world, MSAs
would clean up headers at submission time, and MTAs wouldn't mess
with messages other than putting headers at the top to say where it's
been.
I run qmail, which makes about as clear a separation as possible
between submission and transfer, but even there some kinds of
forwarding accidentally go through the header rewriter.
Perhaps we can say that any MSA/MTA that rewrites headers is taking
responsibility for the message and should be prepared to vouch for
it if it goes out via SMTP again.
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
http://www.taugh.com