I have Thunderbird delete messages on the server when they download, apparently you do not. So whenever you set up a new account you will see a pack rats collection of mail download.

Lord help you if you ever have to pay mobile data charges to add a mail account to a phone.

This is not a new account. The messages are from last year. Why would they be downloading again? These are all messages that I have received before. I also have a web account with the same email addy and they are not loading there. Any ideas?

Thunderbird keeps track of What is downloaded and what is not in a file called popstate.dat If something damages that file or more commonly of late the provider re-indexes the messages on the server and allocates them new reference numbers. Thunderbird is flying blind, what it has is invalid and it has no way of recovering other than starting over.

ATT and Yahoo (those I am aware of) have re-indexed mails frequently over the past year, but there are occasions when a mail provider must do that. Say when they get new mail software from a different company.