Hi Ebasegmez,
Based on your description, the issue is related to Outlook for Mac 15.8 client. Since no server configurations will change the Outlook for Mac 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 client forum which is dedicated to handling Outlook for Mac client related issues.
Your understanding is highly appreciated.
Best Regards,
Erick