I came across this new offering from Microsoft which is on Trial mode today. Evaluate Release 10.1 of Forefront Online Protection for Exchange The new release of Forefront Online Protection for Exchange (FOPE) integrates the support request system with the Admin Console. See the New Features Guide and the 10.1 Admin Center Guide.
This Trial is open for Customers as well as partners. Essentially you need to point the MX records to Microsoft Servers so that all the mails get scanned before they hit your Exchange server.
with warm regards
Microsoft Most Valuable Professional