When I reply to a Sourcehut mailing list from Gnus it creates a new thread reather than adding the reply to the existing one. I’m not the only one that experiences this so you can imagine that us Emacs people will be polluting lists if we don’t fix this.
I can’t tell you what the good, official solution to this is, as I
know little about the inner workings of Gnus and Sourcehut, and
threading seems to work well on other mailing lists, such as the GNU
ones. If you browse the Guix mailing lists you’ll find a lot of
correctly formed threads where emails have the
User-agent header set
to something about Gnus and Emacs. I found a simple, manual hack
Before sending a reply to a Sourcehut mailing list, run
message-insert-headers (C-c M-h) in the compose window. Look for the
In-Reply-To header; it should be of the form of an id followed by
some information in paranthesis.
In-Reply-To: <email@example.com> (Ivan Sokolov's message of "Sun, 23 Aug 2020 10:55:08 +0300")
To fix our problems, just remove whatever is after the id. This is how my example would look after cleaning:
Now you can send it and it should appear as a reply in the same thread as the message you responded to.
Threading has been fixed on Sourcehut and you no longer need to employ any hacks to make it work when you send emails from Emacs.