There is an alternative, and possibly better, way to solve this problem.
Leave the To: and Cc: headers alone, with their present meanings, and let
them be included in all messages as at present.
Now invent ONE new header
no-reply-to = "No-Reply-To" noreplylist CRLF
noreplylist = ( "To" / "Cc" / "From" / "Reply-To" / "List-Post" /
"Bcc" / ... / address) [ "," noreplylist ]
which essentially says, "when replying to this message, kindly do not
reply to any of the people listed". So you can exclude the whole of the
"To:" list, the whole of the "Cc:" list, the whole of the "From:" list
(plus any others that might be allowed) and you can alternatively, or
additionally, exclude any explicitly listed <addresses>.
If you'll note in the draft, I considered something similar to this.
As you point out, it's not compatible with legacy MUA implementations.
It also would arguably have one header field changing the meaning of
another header field. My personal belief is that the "sendmail -t"
problem is a minor one - since it seems to affect a relatively small
number of older MUAs. Others might disagree.
Also, while you are about it, can we include some mechanism for "reply to
list" in mailing lists?
how would this be different than List-Post:?
Keith