I recently completely erased an extra old MacBook I have and installed the latest version of Sonoma (14.6.1) on it to experiment with. I then configured Applemail to work with my email provider (outlook.com), which went fine, but I'm really not a fan of the latest version of Applemail, so I thought I'd try Thunderbird (which I used to use years ago, and generally liked OK). So I downloaded a fresh version of Thunderbird (v128.2.0esr) and configured it with my outlook.com account, which finished the authorization process without a problem. The issue, however, is that Thunderbird will not "see" many of my email folders. I have LOTS of stored email, organized into hundreds, if not thousands, of folders and subfolders. For some reason, about 75% of the folders simply don't appear in the Thunderbird folder tree on the left. I have no problem accessing them via Applemail, but Thunderbird just doesn't seem to realize they're there. The other 25% or so seem to work fine and contain all the messages I expect to be in them. I've checked some of the folders to make sure that some slightly odd folder names (a few start with a hyphen or have a comma in the names) weren't causing issues, but changing the folder names in Applemail or via the outlook web interface does not make them appear in Thunderbird, and some of the folders whose names contain those characters DO appear in Thunderbird, so I don't think that's the issue. If I create a new folder in Thunderbird, it appears in Applemail immediately. However, if I try to create a folder with the same name as a folder that appears in Applemail, the operation fails with the message "The current command did not succeed. The mail server for account ******@outlook.com responded: Folder name is reserved." in the Activity Manager window. If I create a new folder in Thunderbird, then use Applemail to move some messages from a folder that Thunderbird can't see into that new folder, those messages then DO appear in that new folder in Thunderbird. So I assume it's not a problem with the messages that's causing the folders to not be visible in Thunderbird. It really seems that Thunderbird is simply not iterating through the folder structure completely. Since the folders aren't appearing at all in Thunderbird, I have no way to select or deselect them for synchronization for offline use. Is there some way to view Thunderbird's internal retrieval of the folder structure from the server? I've tried restarting Thunderbird many times, including in Troubleshooting mode and clearing the startup cache, but they made no difference. Any help would be appreciated! I'm liking Thunderbird's look and functionality, but I need it to access all my email or I will have to go back to Applemail, or, god forbid, try the Outlook email client :-( Thanks!