Look in account settings to see which folder that Thunderbird is viewing. You have four, which generally indicates there have been problems in the past.

@david Well, all that Message Storage tells me is that the Local Directory is ...\inbound.att.net What to do next? Rename the folder as something else and hope that it goes to the next folder??

Okay, I'm no guru on this, but I suggest highlighting the inbox and rightclicking and checking properties to see if it tells where it's looking. You have four accounts defined and I'm not positive on identifying which one. My awareness is that, if the folders are deleted, IMAP will repopulate them, but yours is so large that it would take a lot of time. The prefs.js file in profile may also identify the active folder. As an aside, you may want to check ll of the folders in windows file explorer to see if there are any garbage files from the prior problem, such as any ending in NSTMP. Such files can be deleted as they are detritus from a failed compaction.

Right-click on Inbox folder properties just points to the location of the folder.

The prefs.js file identifies three of the Mail folders.

user_pref("mail.server.server1.directory", "C:\\Users\\Dad\\AppData\\Roaming\\Thunderbird\\Profiles\\l7fpqsh7.default\\Mail\\inbound.att.net");

user_pref("mail.server.server3.directory", "C:\\Users\\Dad\\AppData\\Roaming\\Thunderbird\\Profiles\\l7fpqsh7.default\\Mail\\inbound.att-1.net");

user_pref("mail.server.server5.directory", "C:\\Users\\Dad\\AppData\\Roaming\\Thunderbird\\Profiles\\l7fpqsh7.default\\Mail\\inbound.att-3.net");

If I were to delete the Mail folders, I don't know where they could be repopulated from, as the mails are long gone from the server.

Okay, I think we can salvage the message folders. Since there are several, I suggest reviewing last update date of each, as they may each contain different messages. To salvage the messages, you could - install the importexport addon if not already done - create a new folder in Local Folders for each of the accounts - highlight each new folder, rightclick and select the importexport tool and select import mbox file and then select one of the inbox files (but not the .msf file) and it will bring the messages into thunderbird. - when done, you will have all the messages locally. my next suggestion would be to delete the account and create again. You will then have messages in the account's inbox, plus the local folders. I do not suggest simply moving them all to one folder, as that may place an overload on the process and leave some messages corrupted. If you really wanted them all as one, you could use the importexport tool to then export each folder as eml files and then click the account inbox and import each as eml files. That would get them all together and then add the addon that removes duplicates and you would be mostly back to normal. This is a common problem for crashed POP accounts. I hope this is helpful.

@david Thanks for taking the time to reply. I'll work on this later today and see how it goes.