In our network, some users cannot connect to either Exchange Server 2007 or Outlook Web Access while others are able to connect to both. On one computer, the error occurred after the user ran the AVG anti-virus update. Some computers return this error message "Task 'Microsoft Exchange Server' reported error (0x800401 ID): The server is not available." Thanks, in advance, for your help!
The problem was caused because our firewall was somehow configured to act as DHCP server, in addition to the regular DHCP server we had. As a result, the computers that received their IP address from the regular DHCP server worked fine. Those that received their IP address from the firewall didn't work!