I can add and send message to MSN account in OCS, however, when those MSN account is not in @hotmail.com format, e.g. @yahoo.com.hk, or @abc.com.
The message sending will be failed, and give these:
This message was not delivered to xxx@yahoo.com.hk because the address is outside of your organization and is not federated with your company, or the address is incorrect. Please contact your system administrator with this information. (ID: 1008)