If you are using Active Directory 2008, you must include the netbios_name or use SMB port 445.

Note: When multiple domain controllers are specified, even if load balancing is disabled, when the load on the primary domain controller reaches the maximum number of connections allowed, new requests are sent to a secondary domain controller as a short-term failover provision, until such time that the primary domain controller can accept new connections.

Note: Password authentication failures are always failures.