A folder isn't synced by default unless it's a folder such as Inbox, apart from when it's opened. Right-click a folder, Properties, and check 'When getting new messages...' to have the folder sync at getting new mail. The purpose of the option is to limit traffic to and from the IMAP server, but shouldn't be a significant factor for relatively small mail accounts.

Sorry, but the way this is presented in Settings, not to mention numerous posts, is very misleading. As I said, I'm using this to ensure I don't lose historical email records in a transition to a new hosting service. If I had just done a cursory check of folder contents I probably wouldn't have caught this and the backup would have been largely useless.

In the course of trying to figure out why this behavior was exhibited I never saw this description of folder synchronization. Furthermore, under all synchronization settings everything is checked for offline use, so this is extremely counterintuitive.

I appreciate the information you provided. I hope Mozilla changes the text in the settings dialogue and help documents to make this clear.

Here is an article on the relevant settings:

http://kb.mozillazine.org/How_do_I_check_for_new_messages_in_other_folders

@sfhowes, thanks for your responses.

Wording to this effect should be used in the settings, and should be in help documentation. I found no links to this document using words like folder synchronization, saving folders, or numerous other searches. Also nothing in any FAQ or Help referenced in Thunderbird's home pages.

None of the extensions or methods mentioned in the article for using this feature as a general setting seem to be supported anymore.

There are also numerous online articles about using Thunderbird to locally backup email archives. None of them that I came across mention this. Not much we can do about that I'm afraid

If you have any pull in this organization perhaps you can have this addressed. Hopefully searching this forum will help others find this thread.

I'll mark this as solved.

The issue of folders not updating unless the box is checked in Properties has come up several times in the past, in this forum, but the relevant topics might be hard to find with the search function. You will probably have better luck with a Google search with this argument: site:support.mozilla.org <enter search query>.