0x80090345 error from Windows 10 mail when attempting Exchange 2003 access

Up until now we have only had windows 7 pro clients connecting to our 2003 exchange server using Outlook 2010 email and public folder access with no problems. Today I have been attempting to get a Windows 10 pro client with it's built in email working with our exchange 2003 but I am seeing an error code 0x80090345.

I have also tried to use Outlook 2010 on the same windows 10 pro client, it automatically connects as a IMAP client where it can then poorly (piecemeal) search and show parts of our public folder store. When I try to get Outlook 2010 on Windows 10 Pro connected to our exchange 2003 then it fails with a message like "The connection to the Microsoft Exchange server is unavailable. Outlook must be online or connected to complete this action"

.... from what little I can determine online the 0x80090345 error and the msg "Microsoft Exchange Server is unavailable" may be the same error. Are they?

What can I do to resolve the problem?

Exchange 2003 is end of life. It probably sounds glib for me to suggest you upgrade Exchange in order to fix your Windows 10 client problem, but... Microsoft doesn't guarantee that new products will work with end of life products.

My googling suggests that perhaps the issue is deprecated security standards. Some complete other third party product is suggesting adding a registry key to fix it, but I hesitate to suggest that based on a quick google.

In short: There may be completely unsupported workarounds, but the supported solution would be to upgrade Exchange. (Sorry.)