You should consider installing Service Pack 2 as soon as possible
Exchange 2003 Service Pack 2 (SP2) was released on the 19th of October 2005. You can download SP1 for Exchange 2003 right here:
Download Exchange 2003 SP2 (109mb)
Required Patches
If you’re running Exchange 2003 with SP1 you should apply the following security-related patches:
April 2005
MS05-021 : Vulnerability in Exchange Server Could Allow Remote Code Execution (894549)
February 2005
MS05-012 : Vulnerability in OLE and COM Could Allow Remote Code Execution (873333)
November 2004
Update for Exchange 2003 (KB 883543)
This Update Rollup resolves problems that were found in Exchange 2003 since Exchange 2003 SP1 was released.