Okay, so other clients can find you and localhost works because it is the host. However, mydomain.com can't connect. Remember that mydomain.com still has to be resolved, routed and connect via the network (not the local stack like localhost) even if it points to itself.
Have you tried NSlookup from the CMD prompt to resolved mydomain.com? Does it resolve to the correct IP address?
Have you tried TRACERT from the server's CMD prompt to see the path that it takes to make the connection? Where does TRACERT stop?
Have you tried CURL/WGET from command line to see their response https://serverfault.com/questions/483754/is-there-a-built-in-command-line-tool-under-windows-like-wget-curl