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)
If you’re running Exchange 2003 with SP1 you should apply the following security-related patches:
This Update Rollup resolves problems that were found in Exchange 2003 since Exchange 2003 SP1 was released.