You just need to setup a new Accepted Domain and a Recipient Policy and add this as the default email address, then the emails will come in and go out as [email protected]:
New Recipient Policy - http://msexchangeteam.com/archive/2006/11/16/431531.aspx
New Accepted Domain - http://technet.microsoft.com/en-us/library/aa996910(EXCHG.80).aspx