| The "From:" and "To:" headers are special, in that they're supposed to
| be set by the "MAIL FROM: ..." and "RCPT TO: ..." commands before the
| "DATA" command.

This is inaccurate, on the protocol level.
The envelope (set by MAIL FROM and RCPT TO) are seperate and
distinct from the From: and To: headers.

SMTP is not really designed for message submission, but it remains
emminently reasonable for a mailer using it for message submission
to set To: and From: seperately.

In any case, though, it is inaccurate to say that those headers
are "supposed" to be set by the commands that set the envelope. They
are in general NOT supposed to be set that way, because they
should have different content, if only the comments/names that
are associated with them.

--jhawk