Author A sends a message to mailbox foo. That mailbox, unbeknownst
to A, is a mailing list mailbox, which is expanded to multiple
addresses (any one of which might be yet another mailing list
mailbox). Although A knows nothing about any of those lists,
clearly the administrator of the list associated with mailbox foo
knows that it expands to a mailing list, as do any administrators
of any such downstream lists.
Yes! Yes! Yes! That is all perfectly true. It is also totally irrelevant.
So please answer me. HOW is the mail expander for 'foo' supposed to know
about the totally separate mailing list 'bar' to which the author
crossposted his message. And if it does not know about 'bar', how can it
possibly arrange for replies to be crossposted to both lists (by
appropriate setting of Reply-To/MFT/whatever)?
It can't. But that's also mostly irrelevant, for several reasons.
1. The vast majority of senders are not going to want to configure their
MUAs to know about mailing lists. (and if you say that they should be
automatically configured, be sure you understand the security implications)
2. Different lists exist for different purposes, and have different
conventions for whether it is appropriate to reply to the list, or to
the author, or to copy other lists on replies. Even if this information
is available to the user agent (and yes, that is a solvable problem),
it's not clear what should happen when replying to multiple lists, and
there is a conflict between the lists' policies.
3. Blind acceptance of MFT when composing replies is a bad idea,
for the same reasons that blind acceptance of Reply-To is a bad idea.
4. MFT doesn't fix the problem of duplicated messages anyway.
Now, supposing some author wants to raise a matter which is likely to be
relevant to both ietf-822(_at_)imc(_dot_)org and
IMAP(_at_)CAC(_dot_)Washington(_dot_)EDU(_dot_) So he
cross-posts to both lists, and ideally he would like all respondents on
both lists to reply to both lists. How can this be brought about?
a) have both list addresses appear in To or CC of the original message
b) recipients use "reply all" to reply to both lists.
What I suspect you really want is to do this without having reply
authors see duplicate messages. The only way to solve that problem
is to have recipients' MUAs or message stores do duplicate detection.
(and yes there are problems even with that if you try to use To or CC
fields to classify your mail. but - while you are free to do so -
this is generally not a good idea anyway. if you want to classify
incoming mail based on whether it went to a list you are much better
off using list-id, return-path, or subaddresses)
This is a hard problem. It is entirely possible it cannot always be
solved. But what is clear is that IF the *author* does not solve it, then
there is NO WAY that the list expanders at either ietf-822(_at_)imc(_dot_)org
IMAP(_at_)CAC(_dot_)Washington(_dot_)EDU can solve it (because they just do
not have the
information to do so).
If you want MUAs to be able to learn information about recipient policies,
use the domains to look up NAPTR records that map the addresses onto
http URLs, and then use HTTP (or preferably, something more efficient)
to retrieve metadata about the addresses. This is very useful, very
flexible, and not limited to lists. And no, it doesn't work when you're
offline but user agents can cache the address information, and/or
pre-load the information when they download mail.