HTTP Authentication Methods

Testing HTTP Authentication Methods for URL https://mail.domain.co.nz/rpc/rpcproxy.dll?cl-cas01.domain.local:6002.
 	The HTTP authentication test failed.
 	 Tell me more about this issue and how to resolve it
	Additional Details
 	Not all the required authentication methods were found.
Methods Found: Negotiate
Methods Required: NTLM

Any ideas how to get around this with the exchange connectivity tester? If I change from negotiate to basic or ntlm, I then have issues with clients on the local network.