What a mess with these headers| So I'll throw in my contribution ;-)
 
The Newsgroup: field defined in RFC850 should be left as Eric said
for it's netnews use. It should contain the owner defined
freeform text which normally would be name of the newgroup and not an address.
 
If we are going to start changing User Agent's and then we could put the
name of the list to some Comment:, X- or other new field just for the users
and maybe User Agent's convenience and not misuse some existing headers.
I remember that the RFC-822 allows for any type of header like Dog-Type:
(quite common in Usenet) so we could define a Listserv-List: field as well.
Mailers rejecting a Moonphase: field are indeed illegal, aren't they?
 
The Reply-To: field is the right place where replies should always
go and never to sender:. If the reply-to doesn't point to the list
the user will have to dig the address of the list MANUALLY from somewhere
so why not a Listserv-list: field. (Hmm. Mailing-List: sounds more generic)
 
Errors-To: should have the highest priority for error messages but
I'm not sure how many mailers recognize it so then one should
use Sender: and If even that doesn't exist (e.g. personal mail) one
would use From: field (On a personal mail the other end might be interested
on the error-reports). On some other networks where the postmaster is
a MANDATORY username on every system one could use it but what to do with
some funny file from a system where the managers haven't even heard of POSTMAST
or INFO accounts.
 
 
Harri