> > I had sent a bug report
 > > (http://debbugs.gnu.org/cgi/bugreport.cgi?bug=38654) about a bug in
 > > messaging in the Emacs 27 branch to the bug tracker over a months ago,
 > > but haven't heard back.
 > >
 > > It's a severe bug: Emacs may send invalid (i.e., unreadable) email
 > > messages to the recipient without the sender noticing. The bug is still
 > > present as of today, and this makes Emacs 27 unusable for me.
 > >
 > > I tried to look into the bug myself, but got stuck somewhere deep in
 > > Emacs internals. Do you have any suggestions for how to proceed or help
 > > fixing this bug?
 >
 > Try describing the bug as part of a frequent/important use case.  The
 > description you posted is so convoluted and full of invoking special
 > functions that it's hard to realize why it is a severe bug that needs
 > to be fixed ASAP.  That other users didn't report anything like that,
 > and no one chimed in to say "me too", the impression of the bug being
 > specific to your setup or usage patterns is reinforced.
Thank you, Eli! I've closed and reposted the bug accordingly. I may well be that
many/most other people do not care (e.g., don't use Emacs for email or don't sign emails).
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=39884