In Deployment: Directory Based Edge Blocking for Exchange Online Protection
What is Directory Based Edge Blocking?
The Directory Based Edge Blocking (DBEB) feature in Exchange Online Protection (EOP) lets you reject messages for invalid recipients at the service network perimeter. DBEB lets admins add mail-enabled recipients to Azure Active Directory and block all messages sent to email addresses that aren’t present in Azure Active Directory.
If a message is sent to a valid email address present in Azure Active Directory, the message continues through the rest of the service filtering layers (anti-malware, anti-spam, transport rules). If the address is not present, the service blocks the message before filtering occurs, and a non-delivery report (NDR) is sent to the sender informing them that their message was not delivered.
Read more at source: http://blogs.technet.com/b/exchange/archive/2014/01/27/in-deployment-directory-based-edge-blocking-for-exchange-online-protection.aspx