Please don't implement support for Mail-Reply-To and Mail-Followup-To
in nmh. Not only are they nonstandard, they're a poor fix for the
problem.
Reply-To is widely misinterpreted as the replacement for the From field
in replies, in such a way that "reply all" goes to Reply-To + To + Cc
if Reply-To is present and From + To + CC if no Reply-to field
is present.
RFC 822 has language that appears to support this view. But a careful
reading of RFC 822 reveals that this prose does not apply to Reply-To with
respect to a "reply all" function, but only with the use of Reply-To
in a "reply to author" function.
This leaves us with the situation where the author of a message is
unable to specify the complete destination for replies. Even if
the author specifies a Reply-To field, if the recipient uses
"reply all", addresses from the To and CC field are still included.
This is the behavior implemented by almost every UA in existence,
but it's almost always the wrong thing to do.
And RFC 822's examples make it clear that Reply-To is intended as
the *complete* destination for replies, not merely a replacement
for the From field.
The right way to fix this is to correctly interpret Reply-To -
not as simply the replacement for the From field in replies, but
as the reply destination preferred by the author of the subject message.
Adding new headers doesn't fix the problem. It only makes the
situation more complex.
--
Keith Moore <moore(_at_)cs(_dot_)utk(_dot_)edu>
http://www.cs.utk.edu/~moore/
Citizen of cyberspace, currently residing in Knoxville, TN, US, Earth.