Eli Zaretskii <eliz@gnu.org> writes:

>  I'm completely confused by the data in this thread.
>  
>  Could you please send a problematic message as a binary attachment?
>  By "problematic message" I mean a message that, when forwarded by the
>  Rmail's 'f' command, produces a problem when the forwarded message is
>  received and read by Rmail.
>  
>  Armed with that information, I can investigate the problem on my end.

    This bug concerns trying to forward an existing message in a Rmail
mbox.  Thus, the easiest way is to start with a known Rmail file.   I've
attached a gziped mbox file; save it and gunzip it.  

    Now, start emacs 28.1 -q, find the mbox file, then do M-x
rmail-mode.  You should have three messages.  Try forwarding any of them
with 'f'; you will see that there are extra >'s in the proposed message
to send; this is the bug.  (There should be no >s before From level 1, 1
'>' before From level 2, etc.)  It is no longer necessary to worry about
the patch for 9521.

    Pictorially, the text we are trying to forward should be:
|From level 1
|>From level 2
|>>From level 3
where the leading |s are to prevent quoting.

    Note that this bug is likely related to #13328
(https://debbugs.gnu.org/cgi/bugreport.cgi?bug=13328).

- Mark