I have a new 2010 Exchange CAS/Hub Transport Server in a mixed 2003-2010 environment. The CAS/Hub Transport is configured with an internal IP address and also has a Public MIP associated with it. I can receive mail but I cannot send outbound. When I send an outbound, for example, to an incorrect ymail account Yahoo will respond back that it is an incorrect address. So it does appear that my email is getting out.