We have been working with Microsoft today to address an apparent outage. The firewall and access rules have been reconfigured and our tests at this moment suggest that the issue has been resolved and mail is no longer being rejected as of 5:30 PM.
Our systems were processing the messages on behalf of our clients correctly and delivering them to the correct Microsoft endpoint, where connection was authenticated and recipient/detail exchanged. At that point Microsoft issued a 4.x.x temporary failure error ("Server busy. Try again later..") and ExchangeDefender continued making delivery attempts with some Microsoft servers issuing a 4.x.x error, some rejecting it with a 5.x.x, and some processing messages correctly. We have been in touch with Microsoft throughout the day and were able to have it addressed. As of 5:30 PM there doesn't appear to be an issue.