I would think Exchange, but a connection error implies either the hostname/IP is wrong, or the port.
Because you would typically get a protocol error if it could see the server but not comunicate. Your machine is implying there is nothing there.
So the scenario is likly to be one of these:
There is nothing (LDAP based) at the port you selected.
Your gateway is wrong so the IP of the server cannot be reached. (If connect via IP)
Your DNS is wrong so the hostname can't be reached (If it is a connectkey model it can't resolve a hostname and will need the IP used instead)