In my new list, I have a user - one of the foundation members, so an important use case - who will not be talked out of using a very old version of Eudora (!) for mail. Replies to threads from that mailer keep sometimes being properly threaded in Hyperkitty and then randomly jumping out into a new thread. This messes up the web view of the threads, and it's not right. Yet the threading appears to be correct in mail programs such as Thunderbird, using the email based interface to the list.
Have others seen such behaviour with ancient mailers? Is it the mailer, or is there a known issue with threading the web interface? Looking carefully at message ID's and how they are threaded to examine the behaviour, I can't see what is causing this. Possibly some interaction with other RFC-822 headers?
Andrew