Pretty much what Danny Angus said:
I strongly prefer a standard which adds mandates that improve
consistency, and mandates existing optional behaviour, rather than
duplicates the optional with a mandatory alternative.
Instead of MAY add some piece of data to an existing header type, make
it SHOULD or MUST instead and already those people who implement the
option will be compliant with that new thing.
An example:
Virtually all MTAs *that send mail directly over the public Internet*
(note the qualifier) already send a FQDN in their HELO(EHLO). Mandating
an appropriate FQDN in HELO is something that many border MTAs already
enforce and would be good to codify in such a standard (which would be
forbidding something 2821 allows). Broad CSV deployment will make this
a MUST on the public Internet whether a consistent
(forwarder/remailer/list) header info standard or the CSV standard
mandate it or not.
<more sniping..>
s/sniping/snipping/ -Freudian slip? :) <joking...>
I see no evidence that the proposed header has any real added-value for
the recipient at all.