Try restart the Thunderbird, then try get messages again!

Thanks for replying. I forgot to mention I had tried that - just tried again. Neither time worked.

Is Thunderbird set to synchronize all older messages? Tools menu (Alt+T) > Account settings > synchronization and storage.

Repair only fixes a corrupt index. Regular appearances of a corrupt index is of itself a serious issue and almost always attributable to third party software accessing the Thunderbird profile to scan, inoculate evaluate or something

There is no synchronization and storage tab for this email. Perhaps because it is POP? Another address I use is IMAP and does have such a tab.

Norton360 is up to date but you are saying I may be harboring something that evaded detection and has caused this? I've used 'repair folder' 2-3 times/year in the past but more like 2-3 times/month the last couple of months.

Any recommendation for finding the culprit?

thanks for the reply

your right, synchronization and storage is an IMAP only thing.

If your mail is still in your web mail, you can force a full download of POP mail by deleting the popstate.dat file from the accounts folder in your profile.

Help menu (alt+H) > Troubleshooting information and click the open folder button.

CLose Thunderbird, open the mail folder in explorer window that opened and then the folder names for the accounts server. Delete popstate.dat and restart Thunderbird. Delete the wrong one and you get more mail that you bargained for, nothing else.

Now to the cause here. I have no definitive answers. It is generic and full of assumptions. Common causes are the quarantining of the mail file by the anti virus program, or an attempt by such a program to excise one email from the file and not updating the index. Concurrency issues, again often involving anti virus trying to scan the file while is is being written by thunderbird. My solution is to exclude the Thunderbird profile folder and it's children from "on access" scanning, leaving scanning to the weekly or monthly scan and ensuring that all folders are compacted and IMAP accounts expunged before that scan, to ensure that email marked as deleted has actually been removed from the file before it is scanned.