This indicates an attempt to exploit a remote-access vulnerability that affects Microsoft Exchange Server.
A remote attacker can exploit the vulnerability by sending an email with a meeting request containing specially crafted vCal and iCal calendar data. As a result, the attacker may be able to take complete control of a vulnerable system.
Microsoft Exchange Server 2003 SP 1
Microsoft Exchange Server 2003 SP 2
Microsoft Exchange Server 2000 with the Exchange 2000 Post-SP 3 Update Rollup of August 2004(870540)
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's web site: