You are referencing the DNS service running on your machine. Two possible solutions. Open Server.app and stop the DNS service. Now, if your machine is doing more than hosting web sites, then this may cause more hard than good. Second option, go to System Preferences > Network. Select your network interface and review the servers listed in your DNS settings. My guess is you will see 127.0.0.1 or your machines IP Address as the first DNS server. Remove this.
The issue is that when your machine asks for www.yourdomain.com, it is asking itself for the location and the answer is your own machine.
A couple ideas
check our /etc/hosts file, you might have an entry for your domain.
sudo pico /etc/hosts
do you see any entries for your domain? If so:
edit the file by navigating with the arrow keys, delete the line
with your domain. Hit ctrl-x when done (enter 'y' to save the change)
Clear your DNS cache
Try a different DNS server, go into your network settings (In System Preferences)
Go to Ethernet or Wifi (whichever you are using), click Advanced and under the DNS tab enter:
Make sure no entries are ABOVE it.
If none of this works, send the results of these commands:
dig +short mydomain.com (replace with your domain)
The results of these should point us in the right direction.