Bob, Does that behavior continue if you restart Thunderbird?

Yes, it does.

what mail provider?

Gradwell. www.gradwell.com

re :IMAP mail account settings, Copies & Folders / Message Archives I have selected "Archives" in Local Folders.

Could you access this area again. select: 'Keep messages archives in' Select: 'Other' and the 'Archive' on Local Folders' option. click on OK

Restart Thunderbird. Try to archive an email. Please report back on results.

This made it work for mail in the Inbox, but mail in subfolders still gets archived on the server, not in Local Folders.

Interesting.... Do you actually have copies of emails stored on your computer or only headers ? I'm wondering if you have mbox files for the Inbox, but not for other folders.

Can you go into offline mode - click on the 'two blue screen icon' located in bottom status bar. When offline it will turn into a two black screen icon. Then attempt to read emails in your Inbox and then select some other folders to see if you can read emails.

Report back on which folders you can read opened emails when in the offline state and which folders only appear to the headers but no content.

Good suggestion. In offline mode I can read emails in the Inbox but not in the subfolders that I tried. Some time ago, I selected some folders to only download headers - I shall reset to download everything and report back.

This sounds like you can archive to a folder of choice only for those folders that have stored emails in mbox files rather than only headers in an index file. Basically you cannot store something in Local Folders if it is not already downloaded and stored in an mbox file. So it cannot be moved out of the imap folders.

I'm hoping this is the case. So after synchronising all folders for offline use, you should in theory be able to use the Archive in Local Folders.

Well, I like the theory, but unfortunately it didn't work. I reset the account to keep messages on this computer and restarted Thunderbird when it had finished downloading all the message bodies. But it still archives messages in subfolders to the server Archives folder and messages in the Inbox to Local Folders. :-(

Test: In offline mode: select an email in a folder (not inbox) and select Archive. does it actually goes to Archive in Local Folders in 'offline' mode?

No, it still goes to Archive in the IMAP account