If you haven’t seen this whitepaper this is incredibly useful to understand the SMTP routing available with Exchange Online and Forefront.

http://www.microsoft.com/downloads/details.aspx?FamilyID=F003109A-3CD6-4372-A0F3-D01BD9F50712&displaylang=zh-tw&displaylang=en

Office 365 Beta documentation: This download includes the documentation for the FOPE connector features available to Microsoft Office 365 Beta participants. It also contains information about known issues with this beta release. For infomation about participating in the Office 365 beta, see http://go.microsoft.com/fwlink/?LinkId=206771.