Allowing Messages to be Sent to Recipients of Internal and External Relay Domains
If you have Exchange 2010 Edge Transport servers in your environment you
probably have Recipient Filtering configured to " Block messages sent to recipients that do not exist in the directory ". This setting configures the Edge Transport server Read More...
Read the complete post at http://www.expta.com/2012/10/allowing-messages-to-be-sent-to.html