"Manual threading" in Thunderbird?
The only solution I can think of is to change the subject-line so that the email would fit into Thunderbird's thread-model (Re: subject). There are two way I can think of:
Forward the email to yourself with a new subject-line.
Use the TB Header Tools Extension to edit the subject-line. However, I have no experience with this extension and cannot guarantee that it works. Apparently, it used to work pretty well with TB2.
[EDIT] The TB Header Tools Extension is not available for current versions of Thunderbird.
I now recommend EditEmailSubject which I use myself with no problems.
Please check a good info on threading here: https://github.com/protz/GMail-Conversation-View/wiki/What-is-threading
Main reason for unwanted behaviour?:
Unrelated threads are grouped together!
Someone was lazy, and instead of composing a new message, just hit "Reply All", and changed the subject. You need to educate these people and tell them not to use reply all when they intend to start a new thread.
I'm still wondering whether there is a way to manually change the reply-to or references headers.
You can use the Header Tools Lite plugin by genius Paolo.
- select message under which to Thread
- message_context_menu>HeaderToolsLite>Change Header Details
- copy Message-ID: (or one of References: to thread randomly)
- select misplaced message
- message_context_menu>HeaderToolsLite>Change Header Details
- paste into References: in comma separated list
- thank