This is a feature of an IMAP connection: it creates folders that mirror the server so you can manage the mail on your server easily from one or more different devices.

To keep only a local copy, you could drag mail into your Local Folders. However, then you would need to look in two places.

Maybe it would be simpler to remove your IMAP account and set it up again as a POP account? Let's see whether a Thunderbird expert can advise on the safest way to do that so you don't lose anything.

Very interesting. Thanks for the info! I was having problems with the POP account & Time Warner (roadrunner) told me to change it. I'm not sure but suspect they may be eliminating POP accounts. Any idea if they could do that?

Mail service providers can pick and choose what methods they make available to you for accessing your mail... so long as you don't get sufficiently annoyed to switch providers.

Do you recall what problem you had with your POP account setup?

I kept getting two errors when I emailed, though not consistently.

(1) The email from address was invalid & (2) The message could not be copied to the sent folder.

In each case, just either rehitting send (in case one) or retry (in case two) solved the problem. By the way, I am still having occasional case two problems.

That is very interesting since POP and IMAP are for inbound email and would have nothing to do with sending. The SMTP server is used for sending and it is usually the same server with the same settings whether you use POP or IMAP for Inbound. More great advice from a provider's helpless desk staff. :)