Your settings look correct, but I'm pretty sure the problem is caused by Bitdefender and/or the VPN. I suggest running Windows in safe mode to eliminate the possible interference of startup apps (like BD and the VPN).

A VPN masks your location, which usually runs afoul of authentication processes. Turn it off, unless there are explicit instructions to allow mail clients to bypass it. BD is known to interfere with TB, as a search of this forum will reveal. I recommend uninstalling it and using the built-in Windows Defender, at least until you figure out how to control BD.

Thanks for your advice SF. I have expunged everything I can find on my laptop about the VPN (Surfshark) and BitDefender. Control Panel/Programs and Features/[right click] Uninstall seems to provide the coupe de grace in both cases (but beware of little booby traps left on the desktop, e.g. MailSpring).

Unfortunately, I still can't get Thunderbird to work. If I select New/Existing Email Account from the 3-stripe TB-menu, I can fill in the server configuration with the existing name, email address and (dubious) password. All looks well until I click on Done, which results in:

• Unable to log in at server. Probably wrong configura'tion, username or password.

There's no option to define a new password.

If I try to create a new email address, I am invited to buy a new one: Mailfence Secure Email or a domain called gandi.net. I tried Mailfence, which asks me to fill in a configuration and then confirm it with a message. But the confirming message doesn't arrive, like anything else currently not on my TB. There is another message box:

• Error with account v**@xs4all.nl: Sending of username did not succeed. Mail server mail.xs4all.nl responded: [AUTH] Unsupported authentication mechanism.

Can you suggest where to go from here?

I think the account wizard is entering the wrong settings. Press the Manual config. button in the account setup window and use imap.xs4all.nl, 993, SSL/TLS, normal password, User name = email address, for the incoming; smtp.xs4all.nl, 465, SSL/TLS, normal password, User name = email address, for the outgoing. Confirm the password by logging in through webmail.

https://www.getmailbird.com/setup/access-xs4all-nl-via-imap-smtp

This is confusing me more and more. The TB Menu item Account Settings doesn't have a Manual config. button, and neither does the item New/Get a New Mail Account which invites me to buy Mailfence (free but gets me nowhere as said above).

However, the web link above takes me to another free email program called Mailbird, which looks rather promising. I'll add to this message if I make any progress with it.

Same problem in Mailbird: it asks for a password when I don't have one, and there isn't a way to create a new password. See below.

I've already lost 3 days trying to get my email working again. Maybe I'll be better off with an older version of Thunderbird. Is that possible?

File/New/Existing Mail Account, enter your credentials, on the next screen click Manual config.

Passwords are created and changed on the mail provider's website. TB only applies the already-established password. Confirm the password by logging into the account on the provider's site, then use it to add the account in TB.

Thanks again. I managed get a new account sorted out with your advice and help of the provider, and their Webmail serves as a good backup. It was now possible to add a new account with the existing messages. I'm not sure how I would do it again;).

That's a nuisance because I tried to archive a block of messages from the new default account to the local folders. I didn't realize that the messages concerned would be deleted from the default account without warning, and I want to keep them in the account.

Can you suggest a way to return some or all of the archived messages back into the default account? It's tempting to tinker with the profile, but I don't want to risk making yet another mess of it!

Please ignore the remarks above. The "archived" items are already back in the default In folder. It turns out that I can simply drag and drop messages, one at a time or in a block, between the archive and the Inbox. At last something self-evident in Thunderbird:)!

I'm not sure why I missed it. Maybe it's because I imported messages from the provider's webmail instead of the seemingly inaccessible Inbox, thereby losing the tags that I rely on. Anyway, I can now safely delete the corrupted accounts.

Warm thanks to sfhows, and also to dotdoc dealing with similar problems (25-07-2022).