We then realized that we had created a new folder called A'poo (without the h) and now we see that that folder / file name is completely missing. ... I did a search in Explorer for the file A'poo.sbd to see whether it exists anywhere on the PC - and it doesn't.

I suppose it doesn't exist in your profile backup either?

Is there a chance any of the missing mail still exists on the server? Did you check the option to 'Leave messages on the server' in your account settings?

here's my question about plan B: re-importing the missing e-mails into TBird,

Sounds great, but how do you import something which doesn't exist anymore?

What about a plan C: can I create a completely new profile that uses my current ATT e-mail address and import all e-mails to that new account

You could, but I don't know what you'd want to achieve by doing that. Also note, your account is POP. Thunderbird can only access Inbox for this account on the server. All other folders you created don't exist on the server, they are all local. That's why creating backups of the profile on a regular basis is so important.

Just checked McAffee - nothing has been quarantined.

McAfee has a very bad track record for causing problems with Thunderbird mail files. It's quite possible McAfee was causing the problem in the first place. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

Thanks for your feedback. To your points:

- the backup profile does not contain the A'poo folder since it was created after the problem occurred - so no luck there.

- yes, I've left all my messages on the server, so that's where my thought for plan B or C comes in. The difference between B and C is: can I just re-import old missing messages (plan B) or if not, can I create a brand new profile that has the same e-mail address and pulls everything in (plan C). On the ATT server, all messages sit in the inbox, nothing has been sorted into other folders - the sorting all happened in TBird.

I realize that I'd have to re-create my TBird folders and sort messages again, but that would still be a viable solution for me.

can I just re-import old missing messages (plan B)

You could, but you'd then possibly have to deal with a lot of duplicate messages. You'd trigger the re-download by deleting the file popstate.dat. http://kb.mozillazine.org/Popstate.dat

can I create a brand new profile that has the same e-mail address and pulls everything in (plan C).

You certainly can. Use profile manager to create a new profile. https://support.mozilla.org/kb/using-multiple-profiles

Then set up your account int he new profile. If you still want to keep messages on the server, make sure to set that option in your account settings prior to accessing your mail on the server.

You can always start the desired profile using Profile Manager. Once everything has been sorted with the new profile, you can delete the old one. Do not attempt to start Thunderbird twice with different profiles, unless you know exactly what you're doing. And remember to always create a backup prior to making changes.

I did set up a new profile, made sure that the account settings still say "keep everything on server" and started downloading old e-mails.

This process works - so thank you so much for taking me through this problem!

Now, the struggle will be to download ~ 20K e-mails - the ATT server freezes after downloading 10-20 messages saying something about RETR command not working. So, I can't sit in front of the computer for several days clicking the "get new messages" button - I'll go nuts ;-)

Will have to think about other strategies to downsize the ATT mail folder - unless you have a suggestion to overcome that problem (it's not a TBird issue).

Thanks again for all you help - Irene

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Does that make a difference?

Among other things, Windows safe mode disables anti-virus software, which may block access to Thunderbird mail files.