I just migrated from Exchange 2003 to new Exchange 2010 server (and will proceed immediately to Exchange 2013; I was just using 2010 as the required hop between 1003 and 2013). Now we are getting NDRs ("The server has tried to deliver this message, without success, and has stopped trying.") back for outbound messages--but only for those addressed to <RecipientDomain>-com.mail.protection.outlook.com addresses (hosted Exchange, right?)
I checked mxtoolbox.com, which revealed no blacklist for my gateway IP address. These messages were not blocked previously (i.e. on our Exchange 2003)--which was behind the same gateway.
Is there something in the default Exchange 2010 setting that the hosted Exchange domains do not like?