This post is over 3 months old. If no one can suggest a solution, can someone at least suggest a diagnostic procedure?

I'm sorry no one has come along to try to address your problem, but that is almost certainly because the contributors here are only available to try to help with Thunderbird issues, and not with server issues. Your problem is a server problem, and you should address your complaints to the "HostEurope" server people.

Thunderbird is an email client, and it is a very good one. But Thunderbird can only display what an IMAP server sends to it. If your accounts on HostEurope don't appear in Thunderbird, the HostEurope server did not respond to Thunderbird's request for the details of those accounts. HostEurope did not send the accounts to Thunderbird. Assuming that you set up the accounts correctly in the first place, HostEurope should be sending your accounts' details to Thunderbird.

This complaint should be directed to HostEurope.

Good luck.

@wisewiz, Thanks for responding, but I cannot agree with your interpretation. If this is a server problem, why do i have no problem accessing these mail accounts from other mail clients? The accounts in question work fine in MS Outlook, Apple Mail (under both iPadOS and MacOS) and Samsung Mail. I have in fact contacted HostEurope, and they referred me (as expected) back here. The fact that the accounts work briefly when I set them up in Thunderbird also indicates that there is no problem on the server side. It is Thunderbird, which forgets the accounts as soon as I close it, even though, as reported above, Thunderbird has created the corresponding folders in its IMAP directory.

I respect your right to disagree with my interpretation. I can't help but wonder, however, why you are not using MS Outlook, Apple Mail, or Samsung Mail, if you have those successfully set up and they successfully retrieve your accounts from the server.

Please do this and report back, just so that we can assure ourselves that you set your credentials up correctly in Thunderbird:

Open Thunderbird and go to Settings (either through Tools > Settings or through the hamburger menu (three horizontal bars near the top right of the interface) > Settings. Choose Privacy & Security, and scroll down to Passwords and click the Saved Passwords... button. Now click the Show Passwords button and then click Yes. The usernames and passwords that you instructed Thunderbird to remember are all displayed. Check them carefully to make sure that you made no mistakes. In particular, the SMTP and IMAP passwords have to be exactly identical for each account. The usernames must be identical, too, of course.

Please let us know what you find.

Also please open Thunderbird with the status bar displayed (View > Toolbars >Status bar or Menu icon > Settings > View > Toolbars > Status bar) and tell us what is shown in the status bar immediately after Thunderbird's home screen appears. A series of reports should be displayed in the status bar at the bottom of your home screen. The more detail you can report, the better.

Thank you.

I respect your right to disagree with my interpretation. I can't help but wonder, however, why you are not using MS Outlook, Apple Mail, or Samsung Mail, if you have those successfully set up and they successfully retrieve your accounts from the server.

I *do* use all of these on their respective platforms, but, AFIAK, none of them are available on Ubuntu, and IAC, I prefer to use the native or default application on each platform I use.

Open Thunderbird and go to Settings (either through Tools > Settings or through the hamburger menu (three horizontal bars near the top right of the interface) > Settings. Choose Privacy & Security, and scroll down to Passwords and click the Saved Passwords... button. Now click the Show Passwords button and then click Yes. The usernames and passwords that you instructed Thunderbird to remember are all displayed. Check them carefully to make sure that you made no mistakes. In particular, the SMTP and IMAP passwords have to be exactly identical for each account. The usernames must be identical, too, of course..

I did this many times before I posted. Even if they were wrong, it wouldn't explain the behavior I observed. According to my past experience with Thunderbird, incorrect credentials should have resulted in an error message to that effect, not in the disappearance of the corresponding accounts *without warning*.

Also please open Thunderbird with the status bar displayed (View > Toolbars >Status bar or Menu icon > Settings > View > Toolbars > Status bar) and tell us what is shown in the status bar immediately after Thunderbird's home screen appears. A series of reports should be displayed in the status bar at the bottom of your home screen. The more detail you can report, the better.

When I open Thunderbird, the status bar displays a series of messages describing how it is connecting to the accounts it knows and its progress in updating from them. When it finishes with that, the status bar is empty (unless you count the "Invitations" count on the right. Of course, it is not attempting to connect to the accounts in question, because it has forgotten them. However, it hasn't forgotten them completely; the "Outgoing server (SMTP)" credential are still there.

Thank you. Something is very wrong about all this, and I haven't a clue what it is, but I am convinced that it is not a Thunderbird malfunction that's causing the problem, especially when Thunderbird is functioning correctly with other accounts.

I hope someone who has a contribution to make will come in on this and solve the paradox.

I'm sorry I couldn't help.

@wisewiz Thanks nonetheless for trying. I appreciate your suggestions and that you took the time to understand the problem! My next step will probably be to set up a test account with HostEurope that I can provide in a bug report, in the hope that someone else may be able to reproduce the problem. Thanks again for your support!

You are entirely welcome. Again, I’m sorry I couldn’t help. Good luck.