>     RFC #  822
> 4.4.2.  SENDER / RESENT-SENDER
> 4.4.4.  AUTOMATIC USE OF FROM / SENDER / REPLY-TO
 
Here is the catchy part:
 
>        o   The "Sender" field mailbox should be sent  notices  of
>            any  problems in transport or delivery of the original
>            messages.  If there is no  "Sender"  field,  then  the
>            "From" field mailbox should be used.
 
What 822 does is in effect bundle two different roles together.
 
Role 1 is the person (or process) that actually transmitted the message.
Role 2 is the person (or process) that handles the notices of problems.
 
I see that in some cases, these roles would need DIFFERENT mail addresses
yet there is no way to specify two different addresses.  One (but not the
only) solution is a new header to account for the difference.  It will
require a CHANGE to RFC822, not just a registered extension.  THAT is what
makes this so hard, and that is why there will be a lot of resistance to it.
 
That assumes that a REJECT-TO field or something like it be used.
 
What if instead we want to have SENDER point to LISTSERV and use something
like NEWSGROUPS?  Again a potential problem exists because we cannot be
sure that an uncivilized mailer will not send something that looks like
a LISTSERV command to LISTSERV.  Even RFC822 admits there are bad mailers.
 
I think the SENDER field is essentially useless as it is (ambiguous).