Another new feature of Exchange Online Protection (EOP) was recently added called Directory Based Edge Blocking (DBEB). With this, all messages directed for email addresses that do not exist in your organization will be blocked on the edge. The message will be blocked and it will not be processed. Otherwise if the message is bound for a valid email address in your organization, the message will continue through connection and content filtering (anti-spam, etc.) policies you have configured in EOP. This allows for more efficiency.
For Exchange Online customers using EOP apparently the change will not be too noticeable. Messages bound for invalid email address was being blocked in Transport rules. With the introduction of DBEB, the block is moved forward in the filtering process. This will be reflected in reporting and there is some planned additional reporting that will be released in the future to differentiate between DBEB and SMTP blocking.
For information about the announcement, please review this - http://blogs.technet.com/b/exchange/archive/2014/01/27/in-deployment-directory-based-edge-blocking-for-exchange-online-protection.aspx.