See if folder index needs repair.

• Right click on folder name and select 'Properties'
• click on 'Repair Folder'
• Click on 'OK'

Interestingly, I generally find the oddball items that turn up in threads months after the discussion ended come from folk that do their mail by finding the last message from you, clicking reply and then clearing out all the visible information so they have a "new" mail.

Unfortunately doing this does nothing to affect the references header, which references the original thread and so when the message comes in, it joins the others from the same discussion. (except it is not the same because the sender either does not know how to use an address book, or can not be bothered.

Toad-Hall, Thanks for the suggestion. Tried it. Did not seem to make a difference. Regardless, I now know about the repair folder command.

Matt, You may be right about why these messages get attached to the wrong thread. Unfortunately there does not seem to be a way for me to fix the threading.

There are limited options that can be set using the [config editor]]

See this rather venerable article http://kb.mozillazine.org/Stop_threading_by_subject

Based on my reading and Thunderbird's current default, you might try setting mail.strict_threading to false. and force Threading by subject and see if it works better for you.

I've never liked threads, but when they don't work properly they become dangerous. I'm trying to sell my late mother's property and could do without Thunderbird hiding an email from the solicitor on the end of a thread of emails from the estate agent {lawyer & realtor respectively in American}.

I found some useful items in Help ('Message threading in Thunderbird - keep messages grouped by conversation'). See the 'Changing default threading mode' section. This says that the procedure described changes the default for new folders.

However, the 'Enable threading on a folder' & 'Disable threading on a folder' sections on the same Help page may need updating. My View menu does NOT have a 'Sort by' sub menu.

After I changed the default for new folders to no threading, I tried Toad-Hall's earlier tip above about Repairing the folder. This repair took account of the revised default and switched off threading for that folder! I'll be repairing most or all folders soon.