I have an exchange server sitting in its own domain behind a firewall. There are no users in this domain but everybody authenticates from an outside domain. I opened up the ports recommended by MS (25, 53, 80, 88, 102, 110, 135, 137, 139, 143, 379, 389, 445, 691, 3268) on the firewall however the outside users are unable to connect to the servers using MS outlook 2k Exchange configuration. They are able to use the IMAP or POP3 protocols. As soon as I open all ports on the firewall to the exchange server they are able to connect as Exchange users. What, am I missing? I understand that one of the ports is still closed when I enable the firewall security but which one?