Keith,
Regardless of the merits of this idea has it been introduced as
part of the original email model, I see several problems. At
minimum, I believe a future version should respond to them. At
worst, it turns this, on balance, into a Bad Idea:
(1) Most of the functionality, for most cases, appears to be
addressed by
Reply-to: address1, address2, address3,...
Now, while that has rarely been well-implemented, it has been in
the standard since 822 or earlier. It is hard for me to see an
argument that a newer set of headers would work significantly
better.
(2) Your legacy discussion fails to discuss the interactions
among a Reply-to field that is present and these new fields if
they are also present. It seems to me that could get
complicated.
(3) To the extent to which this and Reply-to overlap, it gives
us two different ways to do essentially the same thing. That is
rarely, if ever, a good idea.
(4) If these are sent out from a system that has no idea whether
the recipient will honor or ignore them, it seems to me that we
introduce the potential for huge levels of confusion and unmet
expectations. That isn't a showstopper, but argues against
this, IMO, unless it is really clear that there would be
significant benefits.
john