After fighting with Office365 (version 16) and its inability to locate the correct server to process Outlook and other requrests, I found this URL which solved my problem:
Suppress AutoDiscover mismatch warning – Outlook | Microsoft Docs
For reasons unken to me, the RedirectServers entry did not contain the correct value. Following the advice in the article solved the issue.
Apologies if this topic has already been covered!
John