Outlook complains about connection error to AD DC server


we use macOS, Microsoft Office and Microsoft 365. When using Outlook for Mac, every few days or restarts Outlook complains about not being able to establish a secure connection to our domain, e.g. example.com. UCS 4.4 is running as an AD DC.

At the moment, I assume that Outlook tries to get some part of the mailbox configuration from example.com. As we use our domain name publicly for our website and e-mail addresses, the responding servers are different for internal and external access. I’d assume that setting the MX record internally to the same value as the external MX record, Outlook shouldn’t complain, but yet it does.

Does anyone know what Outlook tries to achieve when accessing example.com? The DNS MX record is set to example-com.mail.protection.outlook.com.

Currently, my search yields results in the direction of autodiscover and service connection points. Maybe these terms are known to someone here.

Thanks in advance