This indicates an attack attempt against a buffer-overflow vulnerability in Microsoft Exchange.
The vulnerability is caused by an error when the vulnerable software handles a crafted TNEF context. It allows a remote attacker to execute arbitrary code via sending a crafted mail.
Microsoft Exchange 2000 Server Service Pack 3 with the Update Rollup of August 2004
Microsoft Exchange Server 2003 Service Pack 2
Microsoft Exchange Server 2007 Service Pack 1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's website: