Microsoft Exchange Server is prone to a remote denial-of-service vulnerability. Successful exploits will allow attackers to cause the affected computer to become unresponsive, denying service to legitimate users.
Block external access at the network boundary, unless external parties require service.
Filter access to the affected computer at the network boundary if global access isn't required. Restricting access to only trusted computers and networks might greatly reduce the likelihood of exploits.
Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Deploy NIDS to monitor network traffic for anomalous or suspicious activity. Monitor logs generated by NIDS and by the server itself for evidence of attacks against the server.
The vendor has released an advisory and fixes. Please see the references for more information.