This topic is currently being debated on the exmh, nmh, and gnus list
at the same time. I apologize if you see this more than once. For those
who are just wandering into the discussion, we are talking about the
proposal at:
ftp://koobera.math.uic.edu/www/proto/replyto.html
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.
Well, everything is nonstandard until it becomes standard. The standards
only documented current practice. Since it appears (at least) several
people who develop mail clients are interested in this proposal, it should
be taken seriously.
I also believe this proposal is a good way of fixing this problem. The
problem is the existence or absence of a single header is not enough
information. That is the reason the Reply-To header is typically implemented
in a way that goes against RFC-822 in certain cases.
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.
If every mail client is doing this, then it becomes the standard.
My goal is not to change the behavior of everyone writing mail clients
(that's not possible). My goal is to write a mail client that inter-operates
nicely with the mail clients that other people are using.
Since the Reply-To header is interpreted differently by different people,
fixing this situation is impossible at this time. That is why the proposal
offers two new header fields "Mail-Reply-To" and "Mail-Followup-To".
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.
--
Richard Coleman
coleman(_at_)math(_dot_)gatech(_dot_)edu