bug#9392: 24.0.50; rmail-forward produces messages in unsupported format that cannot be customized away

[Eli Zaretskii <eliz@gnu.org>]

> From: Kenichi Handa <handa@m17n.org>
> Cc: 9392@debbugs.gnu.org
> Date: Tue, 30 Aug 2011 09:36:08 +0900
> 
> > So, given the above input regarding MML, I assumed that message.el
> > does TRT with MML before sending the forwarded message, and tried
> > rmail-forward in "emacs -Q".  However, with that configuration, the
> > code which inserts the MML #part tag is never executed, and in
> > fact the "*unsent mail*" buffer popped by message.el does not have the
> > text of the message being forwarded; the body of the composed message
> > is empty.
> 
> I have not noticed this bug because I seldom do
> rmail-forward; instead I freaquently do rmail-resend (C-u f)
> which still works in the latest Emacs.
> 
> My last changes to rmail.el and rmailmm.el were before the
> release of 23.3, and for 23.3, I surely tested that
> rmail-forward worked with "emacs -Q" (but I must confess that I
> forgot to test the other configuration).
> 
> > AFAICS, the #part tag is not inserted because rmail-forward
> > expects rmail-start-mail return a non-nil value, but with
> > message-user-agent it returns nil.
> 
> So, It seems that something has been changed after the
> release of 23.3.

Can some expert on message.el please help?  E.g., how is forwarding
supposed to work with message.el?

> By the way, I found that if a sending mail contains "<"
> followed by "#part", the following error is signaled.
> 
> forward-sexp: Scan error: "Containing expression ends prematurely", 1099, 1100

Is there some method of quoting/escaping <#part ...> so that MML is
left alone?