For example in this thread which is "Schoology parent profiles - Alex Smith" it also includes the following:

- "Your Children's Weekly Schoology Summary" from a completely different sender that doesn't belong to any other email in this folder. It's also the weekly report, not the profile info, so again a different subject / thread.

- "Cubby - Alex Smith" which is a completely different subject all together having nothing to do with Schoology and recipients then Schoology.

Again thank you in advance.

Usually these sort of odd misthreading come down to either the ignorance of the sender or laxness. Fundamentally each email has a unique ID, it also has a header field called References: When you click reply or forward, the emails ID is appended in the references header, so it is possible that a single email can have hundreds of references. depending on how many replies and forwards it has had.

The issue arises when folk simply click reply on another email that has the recipients they want to communicate with (instead of using a mailing list to set up the recipients) and then proceed to change the subject and replace the body of the email. This does not clear the references field. This is the fundamental difference between clicking reply over "edit as new". This clears the references and changed the message id.

This can also be generated by poorly written web sites and scripts that reuse references and id's.

In both cases it is probably better to ask the sender to stop whatever it is they are doing wrong.

Note that pressing Ctrl+U in Thunderbird will open the message source and you can manually confirm that there is not an issue with the references being read wrong or interpreted wrongly by Thunderbird.

There are some overrides you can set in the config editor. But they might cause other issues for you just as easily as fix things. See http://kb.mozillazine.org/Stop_threading_by_subject

A general discussion on threading is also offered here https://support.mozilla.org/en-US/kb/message-threading-thunderbird#

Thanks for the reply, but this definitely is not a case of someone simply replying to the wrong email. In the example above, the "Cubby" email was a new email I sent, not a reply, and somehow it was threaded underneath a system generated message, which was itself threaded incorrectly. I would never expect a new email I sent would be threaded like this. I checked the headers and the Cubby email does not have a references header:

Content-Type: multipart/alternative;
 boundary="------------5T4oa00np21FChZRYgo2HhGb"
Message-ID: <[email protected]>
Date: Sun, 27 Aug 2023 11:56:01 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.14.0
Content-Language: en-US
To: Rosie Hope --snip--
From: Rachel Wooster --snip--
Subject: Cubby - Alex Smith
Cc: --snip--

I also verified in gmail (my mail provider is Google Apps) and things are threaded there as I would expect. I also checked the settings mentioned in the linked article and I verified "mail.strict_threading" is true, which my understanding of the setting should only thread by appropriate headers. Any other ideas? I've been using Thunderbird for years and this issue has cropped up only in the past couple months.

Thanks for the reply, but this definitely is not a case of someone simply replying to the wrong email. In the example above, the "Cubby" email was a new email I sent, not a reply, and somehow it was threaded underneath a system generated message, which was itself threaded incorrectly. I would never expect a new email I sent would be threaded like this. I checked the headers and the Cubby email does not have a references header:

Content-Type: multipart/alternative;
 boundary="------------5T4oa00np21FChZRYgo2HhGb"
Message-ID: <[email protected]>
Date: Sun, 27 Aug 2023 11:56:01 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.14.0
Content-Language: en-US
To: Rosie Hope --snip--
From: Rachel Wooster --snip--
Subject: Cubby - Alex Smith
Cc: --snip--

I also verified in gmail (my mail provider is Google Apps) and things are threaded there as I would expect. I also checked the settings mentioned in the linked article and I verified "mail.strict_threading" is true, which my understanding of the setting should only thread by appropriate headers. Any other ideas? I've been using Thunderbird for years and this issue has cropped up only in the past couple months.

How do I get the English Page for support?

@tim, You have a topic you have opened here. https://support.mozilla.org/en-US/questions/1428404 so I guess you already know the answer to that question. Please do not post in this topic again.

I don't understand what you mean, I why would I ask the question if I had the answer?

@Rachel, sorry it has taken me so long to get back to this forum.

Can you provide me with the actual EML files for some of these emails {File > save as file (Ctrl+S)} so I can set them up in a folder in Thunderbird and tinker with the entire email thread to determine what is happening. I do not understand it, and really I can't see any recent bugs filed around the subject in Bugzilla. So I will just need to work through an example and try and understand it.