Autodiscover to https://domain/api starting

I have a local installation of Exchange 2016 Server with 4 child domains, each with its own Exchange 2016 Server.

When using the "Test Email Configuration" tool on any Outlook client machine, it shows a non-standard connection. The version of Outlook (2010, 2013, 2016) does not make a difference. This is what I see.

.

Autodiscover to https://server.childdomain.domain.com/api starting

GetLastError=2147954407; httpStatus=0

Autodiscover to https://server.domain.com/api Failed (0x80070057)

Autodiscover to https://domain.com/autodiscover/autodiscover.xml starting

.

Then continues as expected.

.

My question is "Why is Autodiscover calling to the Exchange Backend API folder at all?"

I am sure once I get THAT taken care of the fact an Exchange 2016 Server in a child domain will be a non-issue.

Thanks in advance.