I was not aware Thunderbird had a message that told you your settings were out of date. Are you sure this was not some setting generated by your antivirus program?

So now instead of trying to fix your accounts, we are trying to set them up anew without the original error message or anything to indicate what the original error message was about or what the problem is. If you wanted to make a difficult job harder, you have achieved that.

Almost every instance I have even seen of this sort of error on setting up a new account where the settings are for a major provider that Thunderbird would have detected from it's database and then could not connect to was a direct result of a failed antivirus product, the remainder were VPN related as they do not play well with email.

Start your OS started in safe mode with Networking. Instructions for each operating system: Windows 10/11, Windows 8, Windows 7, OSX

Now try and add your mail account to Thunderbird. Note that a wired internet connection is required for at least Windows safe mode with networking. So plug your laptop in to the router before you try. WiFi will not be available.

This process should load windows without your antivirus, without your VPN and without any of the normal startup programs, many of which can cause connectivity issues.

Hi Matt, Thanks for your prompt response. Firstly, I have deleted and re-added accounts in the past to resolve other problems without any issues. In this case it seemed a good solution to try. But it wasn't and you are correct - I am now in deeper water! I had previously paused my Surfshark VPN when attempting to re-add accounts (couldn't use Mozilla VPN as it did not support Australia). This time when trying again I deleted the VPN from my PC and tried again - but still get the same error message. I use the anti-virus feature of Windows and it is still running. When I first start up Thunderbird I get an error message from "Mail" (which I assume is from Thunderbird) saying my Gmail, Yahoo, etc settings are out of date. After inspecting my Mail Settings, which appeared OK, this led me to delete and re-add my Mail accounts. I have tried adding using Thunderbird's automatic feature, and I also looked up the settings for each account and tried adding them manually, but still get the same unsuccessful outcome. The only other thing to note is that I seem to be able to add my Gmail Account successfully, but when I do it starts downloading hundreds (thousands?) of old emails going back years. I can't see how to stop this other than deleting the Gmail account again. I am wondering if this has triggered a spam control feature that has blocked me? I don't know if this feature exists. So I take on board what you are saying about deleting and re-adding my accounts, and I would be interested in what you recommend I should have done - and what I can do next to resolve this problem. Thanks and regards, Graeme

Hi again Matt, I have some news to report. Firstly, we are travelling at present and accessing WIFI where we can find it. We are currently uing WIFI in a local library. The problem we are experiencing is puzzling so it's time to try some other options. Instaed of using the Library WIFI we hot-spotted to my phone and trieed again. It works, and I can not add mail accounts to Thunderbird. I don't know which part of my system detected a problem with the Library WIFI or what the problem actually is. I will report this problem to them when they open on Tuesday (Australian time). Thanks for your help, and I hope this feedback is helpful when supporting others, Regards, Graeme