Emacs 27 may send invalid email messages without signaling an error

[Rainer Gemulla]

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?