TBird downloads same messages repeatedly.

I have seven e-mail accounts in TBird, all on the same domain that I own. Mail is hosted by HostGator. My most-used account redownloads the same messages repeatedly. The account checks for mail every minute but the redownloads happen only 5- to 8 times a day. This is happening on only this one account and no other.
Things I've tried:

• deleted popstate.dat so it would rebuild on next use of TB
  
• deleted inbox.msf so it would rebuild on next use of TB
• uninstalled TB, deleted all TB related files and folders on the computer, restarted the computer, then reinstalled TB from scratch. I transplanted NO files from the old installation.
  
• deleted popstate.dat again, and inbox.msf.
• verified that I am deleting the proper popstate.dat and inbox.msf files for the troubled account.

Help!

 

Thanks for reading and your reply.

It is POP3.

Short answer is not since I did this: After posting here, I deleted that account using HostGator's control panel and then recreated it. So at present there is no incoming mail there. I will know if this latest desperate attempt has produced any change maybe in the next 24 hours.

I've been having multiple TBird problems, mostly mail sporadically not being delivered or being delivered anywhere from a few hours to as much as four days late. I worked on that off-n-on for two weeks and it appears the delivery problem went away on its own as nothing I tried produced a change. As soon as the delivery problem went away, this repeating problem appeared.

 

TBird just now crashed and Mozilla Crash Reporter came up. I restarted TBird and it re-downloaded all the same mail from yesterday.

 

Update: Crash Reporter crashed also.

 

Yes, ad infinitum, all suggesting pretty much the same couple of things which I have already done many times and they don't work.

Today, I started TBird and discovered that my messages in inbox are set to Threaded again. That's another thing that I have to set daily back to unthreaded. To be clear, this happens all on its own and only in this one account.

 

New development:

I started TBird this morning and checked mail on all accounts and got the following on all messages that were supposed to be in the troubled account's folders:

File not found
The file mailbox:///D:/AC/Thunderbird/Mail/mail.xxxxxxxxxxxx.xxx/Inbox?number=8 cannot be found. Please check the location and try again.
Check the file name for capitalization or other typing errors.
Check to see if the file was moved, renamed or deleted.​

Using Windows File Explorer, I looked and found only two files, Inbox.msf and Trash.msf. Inbox, msgFilterRules.dat, popstate.dat and Trash were all missing.

I suspect a virus or a remote hack as the same files are missing from a daily XCopy-type backup on drives E, X and Y (see attached image). I don't expect a virus would do this and target those backups on all three drives. In any case, it looks like my next few days will be tied up formatting and rebuilding.