AWS Planning and Implementation Guide for Microsoft Exchange Server http://aws.typepad.com/aws/2013/07/aws-planning-and-implementation-guide-for-microsoft-exchange-server.html

Do you have rogue Internet gateways in your network? Check it with nmap http://isc.sans.edu/diary/Do+you+have+rogue+Internet+gateways+in+your+network?%2BCheck%2Bit%2Bwith%2Bnmap%2F16198