This is a patch for nnmail.el.  As you can see, it's been created for
the mid sixties, but it still works.

What does it do?  It stores additional information in ~/.nnmail-cache;
in addition to the message id the group name is stored as well (for
mail groups only).  It also defines a function to be used in
nnmail-split-fancy which looks at the References header of each
incoming message and searches the message ids in ~/.nnmail-cache.
When a message id is found, the group name next to that message id is
used as the group to split to.

This is a complicated explanation.  Let me try to illustrate this with
an example.  Let's say you receive a message M which is split to
nnml:mail.misc by your fancy splitting rules.  Let's say you then
*manually* move this message to some other group, nnml:mail.foo.

Suppose someone sends you a followup F to the message M.  Surely you
would like F to be split to nnml:mail.foo, as well.  But the fancy
split methods don't know about the manual movement, and thus they
would choose nnml:mail.misc.

Well, with this patch the followup is also split to nnml:mail.foo.

,-----
| (setq
|  nnmail-split-fancy
|  '(| (: nnmail-split-fancy-with-parent)
|      ("x-mailing-list" "bbdb-info@xemacs.org" "auto.bbdb")
`-----

This is the beginning of my nnmail-split-fancy value, as an
illustration how to use the whole thing.

I would like you to test this patch if it also works for you.  Beware
of nnmail-split-fancy putting stuff in the bogus group if there is an
error.  Other than that, this should be benign.

I've got an agreement with Lars.  He wants to rewrite the mail
fetching stuff (he's already started to do so with mail-source.el),
and then the patch will break.  But if we discover that the users
(i.e. you) think that this behavior is a nice thing to have, maybe
Lars will build it right into the new mail fetching stuff.

(In case you're wondering: currently, the first backend which can
receive mail gets all of it.  Lars wants to allow one to specify which
backend a mail should go to.)

Let me know what you think,
kai
-- 
I like _b_o_t_h kinds of music.