Thanks, all, for the replies.

After speaking to Kovich in unicast, I realized I needed to explain the issue in more detail.

When we ran Exchange on-prem or in the cloud, there was no issue running macOS's native Calendar app with it. However, when we moved to the Office 365 cloud service, it is a whole other affair with how Microsoft offer that service compared to their generic/previous cloud Exchange.

With Office 365, non-Microsoft apps have to be pre-approved by Microsoft, at which point they can be loaded into the master profile for your enterprise account with them, e.g., Thunderbird, e.t.c.

This all became necessary after Microsoft (and other cloud providers) deprecated/favoured "Normal Password" authentication for OAuth2 authentication. In Microsoft's case, it was a full-on deprecation.

Google have the same feature for their cloud services, something they call "Less secure apps". However, Google seem to be more generic about allowing non-Google apps to access their cloud vs. Microsoft who need to pre-approve 3rd party apps that you can add to your enterprise profile. Well, at least as far as I can tell.

Microsoft call it "Admin Consent", or something like that:

    https://learn.microsoft.com/en-us/azure/active-directory/manage-apps/configure-admin-consent-workflow

Thunderbird, and as far as I can tell, iOS in general, are supported. So I can use Thunderbird to read e-mails hosted by Office 365, because that is a 3rd party app Microsoft support and that your 365 admins. can authorize. There are a ton of other 3rd party apps Microsoft support on 365 from a multitude of other developers.

However, macOS's native Calendar app is not one of them. This surprises me, which is why I reached out.

A link of what pops up on the macOS Calendar app (and other non-Microsoft apps), looks like this:

    https://ibb.co/XtvfpJ8

I realize that how Office 365 works on the back-end is probably foreign to a lot of people (I know it is for me), but hopefully there is one person here that knows enough about this to point me in the right direction, as our own 365 admins. are stumped.

Mark.