It is an Exchange Active Sync issue. In the mail app it is caused by using the server eas.outlook.com which is found under settings account.
I have not been able to find a way to resolve the issue, as because my account is tied to my Office365 subscription the app will not allow the server to be changed.
There are various other users / posts identifying a similar problem, with many of them having contacted Microsoft Support. However a resolution has not been identified.
This appears to be a issue where we have to wait for Microsoft to get it together and resolve the server issues at their end.
In my case, deleting the mail account after switching to a local account made absolutely no difference.