*Update*
As of yesterday afternoon we are no longer being blacklisted by Microsoft.

Previous Description (2013-10-23 15:50:50):

Due to a compromised email account one of our 3 mail routers has been blacklisted by Microsoft. Some outgoing mail has been rejected this afternoon.

We have taken steps to fix the issue and we should not see any more blocked messages, you can resend any blocked messages and they will go through now.

IT Services will update this issue when all has been resolved.