I have Cforms2 working with my internal Exchange 2007 server, but I would assume the configuration should be similar for a hosted Exchange solution. If you had Exchange on premise, you would need to setup a Receive Connector on your Hub Transport to allow un-authenticated connections from a specific IP or range to pass email through the server.
For your hosted Exchange, I would suggest that you contact their tech support and ask them if they will allow and/or configure SMTP relay from the IP address of your WP server.