Hi AlanTurnley-Jones,
Based on my experience, the issue is related to the Outlook for Mac client. You may try the method below: (Some users' issue is fixed by this way, but the same issue may have different root causes. So this method may not work for you.)
1. From the Ribbon, Go to Tools -> Sync Errors.
2. On the pop window, click on the settings gear at the top right
3. Select "Turn on Logging for troubleshooting" and click OK.
4. Restart Outlook (you will see an annoying prompt when you start Outlook. Please select to keep Logging On).
5. To check whether there is any improvement.
Since no server configurations will change the Outlook client behavior and this forum focuses on Office 365 cloud service related problems, we would suggest you post the issue in our Outlook for Mac forum which is dedicated to handling Outlook for Mac client related issues.
Your understanding is highly appreciated.
Best Regards,
Erick