In the Thunderbird window you should see a list of your messages and to the left, in a separate pane, a list of your email folders (inbox, sent, deleted etc.). Please right-click on the folder that is affected and select 'properties' from the drop down menu. On the 'General Information' tab you should see a button labelled 'Repair Folder'. Please click that button, then close and restart Thunderbird. If that doesn't work, post again in this thread and we can suggest other things for you to try.

I had sent a message back to manchester man to notify you that this method (I tried it three times to be sure) did not work and I still have the original problem. Unfortunately, the mail was undeliverable and it came back. Please help with other suggestions. Thanks!!!