Configuring a WCF service across security boundaries can be a tricky business, or so I learned recently. Testing went well, but the move to production failed for a WCF client/server scenario, with the client application encountering an error: SOAP security negotation with '<myEndpointAddress>' for target '<myEndpointAddress>' failed. See inner exception for more details. Inner exception: The Security Support Provider Interface (SSPI) negotation failed.