What have you got set up in the Account Settings?

Right click on pop mail account name in Folder pane and select 'Settings' select 'Copies & Folders' for pop mail account. see image below as guide. Message archives: select 'Keep message archives in' select 'Archives folder on: pop mail account name

Do you have the above selected?

Have you set up 'Archive Options' ? click on 'Archives options..' button Choose prefered option. If you want to keep folder structure, select the checkbox. click on OK

click on OK

Thanks for the response, but I don't think it helps much. I have had the settings like you describe for some time (that seems to be the problem):

If one sets up Archive options as you explain and that folder does not exist (as seems to be the default setting in Thunderbird) one occasionally gets the message "should old messages be Archived?" (or something like that) but the messages seem to be deleted rather than archived (as the "Archives" folder doesn't exist with pop accounts unless one creates it manually). Once one creates the folder, only newly archived messages appear (there seems to be no check when switching archiving on whether the "Archives" folder actually exists.

Or are the archived messages being moved to another directory?

Any answer would be gratefully appreciated!

Perhaps they were moved to an Archives folder in the Local Folders account?

Sadly, no - but thanks for the suggestion!!

I also looked through all the "Deleted" folders also but couldn't find them....

Did the settings on the POP-based account not hint at where the Archives folder would be located?

In general, Thunderbird has several folders that may not appear until they are needed. I am pretty sure that Templates and Drafts operate like this. I'd kind of expected this to be the case for Archives too.

In general, with IMAP-connected accounts, I might want the Archives to be outside the IMAP account, and therefore not on the IMAP server. In this case I'd want to create them in Local Folders, but ISTR that it doesn't always offer this option.

I'm afraid I don't use POP routinely and so can't confirm your suspicion that Archives may be handled differently in a POP-connected account.

If you have an idea about some particular message or topic that has been archived, the global search may be able to tell you where it is. You'll get a fancy list of its discoveries; use the "show email as list" button to display the found messages in a conventional list format, where you can use "open message in folder", or add Account and Location columns to discover their whereabouts.

Thanks ... I searched everything I could

I know that some folders are automatically created 'serverside' (and then mirrored client side) with IMAP. An IMAP account may have been 'assumed'.

As this seems to be a kind of bug (deleting messages when the user thinks they are being safely archived somewhere) then maybe I should post it somewhere else...?

You said the 'Archives' folder was not auto created in apop mail account.

I use pop mail account as well as imap. 'Archives' folder was not created until I selected to Archive. In the same way 'Sent' folder was not created until I sent my first email.

What do you have in the Profile folder? Make hidden files and folders visible. http://kb.mozillazine.org/Show_hidden_files_and_folders

In Thunderbird 'Help' > 'Troubleshooting Information' Click on 'Open Folder' button

a new window opens showing contents of Profile name folder.

As you are talking about POP accounts:

Click on 'Mail' folder click on the pop mail account name Please Post an image showing all the contents of that folder, so we can see what is going on.

Can you confirm that you have the following in the Account settings for the pop mail account.

select Server Settings then click on the 'advanced' button What is selected? Have you got 'Inbox for this servers account' selected? See image below.