What mail provider are you using?

Hello, Matt. The e-mail provider is compuserve.com

Are you thinking this is a problem at the provider's end?

Compuserve is now part of Verizons Oath inc, along with AOL and Yahoo. You are not the first Compuserve/AOL user in these forums complaining of id=ssues that used to be exclusively the realm of Yahoo. I therefore assume everyone is being migrated behind the scenes to what I see as the seriously flawed Yahoo facilities and servers.

Yahoo have always played fast and hard with the index numbers on mail, re-indexing a couple of times a year at some points. Such re-indexing causes the carefully crafted list of downloaded messages Thunderbird stores in the popstate.dat file to be useless. The index numbers it has are now meaningless. The result is everything on the server becomes "new" again as it has a new index number. Much as I hate to say it, you really have only a few choices to avoid the frustration.

1. Switch your mail to IMAP, then at least the changes will remove the duplicates.
2. Stop leaving mail on the server.
3. Leave mail on the server for only short periods (days or weeks) so the numbers of mails tat are duplicated are less and more manageable.
4. Get a mail provider that actually values their customers. I have created an account with GMX.com as they ask very little, offer a standards compliant service and do not track you, or profile you, as US based corporation do. Something about being subject to German law. They know about privacy like no other really.

Thank you, Matt, for you detailed analysis and very helpful suggestions. I will evaluate my options and may reply later if I have any follow-up questions.