I did not realize this was a public support page...I thought it was for Thunderbird help so I apologize for the lack of information. I was waiting for them to reply with questions that would be helpful in diagnosing this issue. I have my Thunderbird set to synchronize all messages and not to delete any. When I was viewing my emails yesterday morning, all of a sudden all IMAP emails that were local (not on the Bellsouth server) disappeared. It looks like the ones I moved to folders are still there, but the ones in my inbox are gone. I have been able to see them until yesterday and it looks like the last Thunderbird update was done on 9/7. I don't have any add-ons. Not sure why they would have just disappeared. I was sorting emails by subject, then by date and then they were gone. Does anyone have any idea why this might have happened? Thanks for your help!

When using IMAP, what you see under your account in Thunderbird is exactly what is on the server. So, the Inbox in Thunderbird is a view of the Inbox on the server. Messages that are not on the server will also not be seen in Thunderbird. That's how IMAP works. What you see in the client is what exists on the server. What happened to the messages on the server? Did you delete them on the server hoping to retain the copies downloaded in Thunderbird? If so, then IMAP is NOT the right protocol to use in Thunderbird. See The difference between IMAP and POP3 and IMAP Synchronization

Thank you for your reply. That is confusing to me because the setting on the server has always been to delete messages older than 45 days. But, I have been able to see the messages older than that in Thunderbird until the other day. All I did was sort the inbox by subject, then again by date and the older emails just disappeared. Not sure how I was able to see them then.

The articles I linked to are pretty straightforward to understand, but if they still don't make it clear how IMAP works, then a quick search of "IMAP vs POP3" on Google will lead you to more articles or videos on the same subject that will help clarify things even further. It is important that any user of an email client like Thunderbird understands what it basically means to use either POP3 or IMAP. It is time well spent, then you can make a wise decision when choosing which protocol to use to access your email accounts.

I can only think of two reasons why you could access messages older than 45 days in Thunderbird:

1. The server is not doing what it's been configured to do.
2. Thunderbird is not syncing with the server when it should.

The first case is highly unlikely, though possible. The second case is highly likely and possibly what happened in your scenario. Thunderbird may have not communicated with the server soon enough for the older messages to "disappear". Maybe Thunderbird was running in offline mode, you're having internet connectivity issues or something else, such as antivirus or other security program is interfering with Thunderbird's communication with the server, but as soon as that communication was successful, Thunderbird finally replicated (mirrored) what had already happened on the server (deletion of messages). It just so happens that it did this when you were sorting messages. That's just a coincidence, because it could have happened while doing anything on your computer.

Thank you so much for your help. Much appreciated!