Hi,

nowadays, many people don't a have properly configured system, so that
`system-name' often isn't useful for the generation of the domain name
of Message-Ids.  Additionally, many people have the own FQDN
(e.g. from their provider, like CIS-DFN) or the have their own domain.

People often ask how to set the FQDN[1].  They are often advised to
redefine `message-make-fqdn' or `message-make-message-id' (ugly hacks,
IMHO) or to set `mail-host-address':

,----
| (defun message-make-fqdn ()
|   "copy docstring from orig def"
|   "hotmail.com")
| (defun message-make-message-id ()
|   (concat
|    "<" (message-unique-id)"@some domain name You like>"))
`----

But there only very rough checks in `message.el' concerning the
validity of this string (does it contain a dot?
localhost.*?). Ref. [1] also is a nice example about invalid domain
parts: "@a.z«". :-(

I propose to add a variable `messsage-user-fqdn' *and* add a better
validity check for the domain part as well.

,----
| 2003-02-20  Reiner Steib  <Reiner.Steib@gmx.de>
| 
| 	* message.el (message-user-fqdn, message-valid-fqdn-regexp): New
| 	variables.
| 	(message-make-fqdn): Use it.  Improved validity check.
| 
| 2003-02-20  Reiner Steib  <Reiner.Steib@gmx.de>
| 
| 	* message.texi (News Headers): Update description of Message-ID.
`----

The new variable `message-valid-fqdn-regexp' is a duplication of
`gnus-button-valid-fqdn-regexp'.  But AFAIK, it's not allowed to
require `gnus-art' from `message.el', is it?  What about the other way
round?  Other possibilities?