Using older versions of Outlook for Mac in business environments
18 Macs on 10.12.4 and Mac Mail still has the same issue. Setup mail.domain.com as the mail server and it complains that www.domain.com doesn't match mail.domain.com.
This happens because it's looking up "domain.com" and in our case *.domain.com points to the server holding www.domain.com.
Not everyone will have this configuration and it's a major F Up on Apples end.