Are you getting a too many hops error message from your Microsoft Exchange server?
#5.4.6 SMTP; 554 5.4.6 Too many hops
The default setting is 30 hops. It requires a lot hops around the internet and a variety of "interesting" mail environments to take a legit path that is over 30 hops.
Before increasing your hop check above 30, please make sure that the message isn't looping and/or that it's impossible to reduce the number of hops the email is following before it can be delivered.