Since some time ago, I've noticed that my Thunderbird email client was not nesting Moodle forums correctly, with the 1st message in every discussion falling apart from the rest.
Specifically I've noticed that today in the partners.moodle.com forums, but I think I've got the same also from other sites, just cannot check them now. moodle.org forums are working ok:
Looking to partners emails I've seen that they get the header overwritten or so and they are like this:
And that makes In-Reply-To/References to stop matching:
So nesting stops working properly. Only works by subject matching that is really limited.
We should try to make mail systems to stop overwriting those headers, so nesting works nicely.