I’ll note that a number of groups and forums send mailing list like emails (google groups, django dev being a big one) and that notifications can be threaded from places like Github with the right client.

Thunderbird has good threading.

Roundcube webmail is also capable here. Though when I have had it working it didn’t include sent messages… which is not great in my mind.

Thunderbird has good features for mailing lists and threads.

Sounds like you’re into guilty until proven innocent, which is pretty warped.