Why message tracking cookies should be in the message headers

I was asked to post the following to the mailing list at the WG
meeting.

There is one good reason that I can think of to put message tracking
information in message headers instead of (or possibly in addition to) 
an SMTP argument.  If we enable delegation of tracking (as current
proposals seem to allow), the message recipient could track backwards
after the message has been received.

Tracking information can be sent through systems that may not accept
it in SMTP.

I can't claim that this behavior is necessarily useful.  I haven't
been able to think of a really strong reason for it in the last week.
If the message has reached its destination, all the useful tracking
information you're likely to find is in the Received headers.  If the
message hasn't reached its destination, you can't track it backwards.

This leaves only two reasons for this approach: one, it enables users
at both ends to do something that may amuse them; two, we may think of
something clever later that involves going through an uncooperative
MTA.

If a message goes to multiple recipients with different cookies for
each recipient, this method doesn't work.

Tim