This indicates an attempt to exploit a Denial-of-Service (DoS) vulnerability in Microsoft Exchange Server.
There is a vulnerability in some versions of Microsoft Exchange Server that allows an unauthenticated user to connect to the server and issue a specially crafted request which can cause a Denial-of-Service or the execution of arbitrary code.
Any unprotected Microsoft Exchange Server 5.0, 5.5 or 2000 is vulnerable to the attack.
Attackers may cause DoS or execute arbitrary code on the victim machine.
Apply patches or upgrade the server to the latest non-vulnerable version immediately for any vulnerable server.