Is this a pop mail account? Have you selected the 'Account Settings' > 'Server Settings' option to 'leave messages on server' ?

If yes, to both questions: It is possible the pop mail account popstate.dat file has got corrupted. The popstate.dat file keeps track of which messages have been left on the POP3 server. No worries, you can delete it and a new one gets created. Drawback: TB will no longer know what it has previously downloaded and so will download anything in server Inbox, but it should only do this the once. Although, I believe gmail may act differently and only download the last 30 days.

How to delete the mail account 'popstate.dat' file: To help with identifying old mail, I suggest you move all good wanted emails out of Inbox folder into other suitable folders for organising and storage. Delete any emails you do not want. When Inbox is empty, right click on Inbox and select 'Compact'. Now you have a fresh clean Inbox.

Make hidden files and folders visible:

• http://kb.mozillazine.org/Show_hidden_files_and_folders

In Thunderbird

• Help > Troubleshooting Information
• click on 'Show folder' button

a window opens showing the contents of your profile name folder.

• Close thunderbird now - this is important
• click on 'Mail' folder
• click on pop mail account name
• Delete 'popstate.dat' file
• close window - top right X
• Restart Thunderbird.

A new popstate.dat file will be created and should start downloading a load of emails. When it has finished. Delete all of those old emails, only keeping anything that really was new. then right click on Inbox and select 'Compact'. It will remove all of those 'marked as deleted' emails and reduce the file size.

Thanks for the clear explanation. I cannot get to the window Folder Options to mark hidden files because I do not remember how I tried to download athe new versiion of Thunderbird and I get the error message Mozilla is questionable. Will that help me. To stop the 1000s of repeat d=emails I had to change the settings to send them to a non existing email acount. Thanks Judy

re :I cannot get to the window Folder Options to mark hidden files because I do not remember how... The link gives full details with images. The reason I say to make hidden files visible is because Appdata foldres and files and not always visible, hence make them visible.

This 'Folder Options' is not found in Thunderbird as that is a program you downloaded. You need to Open any Windows OS window eg: 'Control Panel', 'Documents' folder. There will be either a 'Tool's > 'Folder Options' AND/OR 'Organize' > 'Folder & Search Options' on the toolbars.

Please note: If the above 'popstate.dat' info fails then this could be related to your Anti-Virus product, in which case, access AV product and stop it from scanning incoming mail. It will still scan any attachments you open.