If Internet Explorer is unable to navigate to a web site and issues the following error 'DNS server error or unable to find server', then the problem may be the following:
1) A web site URL such as http://mail.douglas-associates.com is added to the Trusted Sites list in IE.
2) The IP address of the given web site changes.
3) When entering the URL (http://mail.douglas-associates.com) in the address bar, IE will issue the error indicated above. If the new IP address is entered, the site comes up.
To fix the problem:
4) Remove the URL from the Trusted Sites list in IE.
5) Exit IE.
6) You may add the site back to the Trusted Sites list.