Emails from the moodle.org forums are not threading properly. I thought this might have been a regression of: MDL-52997 but it looks like an AWS issue.

eg in this thread:

https://moodle.org/mod/forum/discuss.php?d=395764

You can see from my first email header:

Subject: Moodle in English: Proposed adoption of mod_groupselect
Message-ID: <0101016fa75a4c94-38bfe565-70ea-40c7-b321-c4c2368911a3-000000@us-west-2.amazonses.com>
List-Id: "Plugins traffic" <moodleforum1190@moodle.org>
...
List-Unsubscribe: <https://moodle.org/mod/forum/subscribe.php?id=1190&d=395764>
Thread-Topic: Moodle in English: Proposed adoption of mod_groupselect
Thread-Index: 646082d990334df2b392e5aa6e91

and then in the later emails:

In-Reply-To: <646082d990334df2b392e5aa6e91a5d72b80c257bc8224d8e98c10bafbdc4820@moodle.org>
References: <646082d990334df2b392e5aa6e91a5d72b80c257bc8224d8e98c10bafbdc4820@moodle.org>

Emails are not grouped, it will mostly affect the first email in the thread:

I'm assuming moodle.org sends via AWS, and AWS is rewriting the message IDs from:

646082d990334df2b392e5aa6e91a5d72b80c257bc8224d8e98c10bafbdc4820@moodle.org

to

0101016fa75a4c94-38bfe565-70ea-40c7-b321-c4c2368911a3-000000@us-west-2.amazonses.com