I have a WCF service hosted in a Windows service. The application is an intranet app, and I have programmatically set the bindings on both the service and the client as: NetTcpBinding aBinding = new NetTcpBinding(SecurityMode.Transport); aBinding.Security.Transport.ClientCredentialType = TcpClientCredentialType.Windows; aBinding.Security.Transport.ProtectionLevel = System.Net.Security.ProtectionLevel.EncryptAndSign; Both the service and client have endpoints configured with SPNs: EndpointAddress = new EndpointAddress(uri, EndpointIdentity.CreateSpnIdentity("Service1")); As far as I know, I have setup the bindings correctly-- and I am usually able to connect to the service just fine. I did however run into a case where on a server running Windows Server 2003 R2, x64, SP2 I get the following exception immediately when the client tries to connect: INNEREXCEPTION -- Exception Message: InvalidCredentialException: Either the target name is incorrect or the server has rejected the client credentials. Stack Trace: at System.Net.Security.NegoState.ProcessAuthentication(LazyAsyncResult lazyResult) at System.Net.Security.NegotiateStream.AuthenticateAsClient(NetworkCredential credential, String targetName, ProtectionLevel requiredProtectionLevel, TokenImpersonationLevel allowedImpersonationLevel) at System.ServiceModel.Channels.WindowsStreamSecurityUpgradeProvider.WindowsStreamSecurityUpgradeInitiator.OnInitiateUpgrade(Stream stream, SecurityMessageProperty& remoteSecurity) I get the exception when I try to connect to the service from another machine in the domain, but if I connect to the service on the same machine running the service it works fine. The hosting service itself is running as a domain user account-- but I have tried running the service as a Local System and Network Service to no avail. I have checked the Local Security Policies for the server and didn't see anything amiss (i.e. 'Access this computer from the network' includes 'Everyone'). Anyone have an idea of what could resolve this? I am wondering if I need to do something in Active Directory with respect to the service's SPN? I have read some about using setspn.exe to register or refresh SPNs, but I haven't needed to do this before. Why would this be working with other configurations but not the one above?