Exchange Servers are resource intense machines that can be improved with some small but very important steps. Applying a few, or better yet all, of these tips you can increase the performance of your Exchange 2003 server(s).

Lets get right to it!

1. Use Windows Server 2003 Exchange Server 2003 was designed for Windows Server 2003. This allows the memory tuning, processor optimization, and other performance tweaks in Windows Server 2003 to be utilized by Exchange 2003. 
2. Memory and CPU Specifications Exchange Server 2003 is a 32-bit application and therefore can only access up to 4GB of RAM, any more is a waste. This effectively limits the number of CPUs to 4 as the memory bottleneck will negate any performance increase from more CPUs. 
3. Hyper-threading or Dual-Core CPUs Windows Server 2003 supports Intel's Hyper Threading technology and it can boost performance by up to 25%. Hyper-Threading Technology allows multi-threaded software applications (like Exchange) to execute threads in parallel increasing performance. 
4. Use Cached-Mode with Outlook 2003 Using Outlook 2003 with Exchange 2003 brings a new feature into the mix, Cached Mode Exchange. With this feature enabled, a copy of the users mailbox is stored locally the two mailboxes are sync'd. This reduces network traffic dramatically. 
5. Configure the Boot.ini file If your Exchange 2003 server has more than 1GB of RAM add the following switches to the boot.ini file. /3GB /USERVA=3030 
6. Place Anti-Spam and Anti-Virus Software on Gateway Devices One of the biggest improvements can be made by blocking Spam and Viruses on a gateway device or relay server as opposed to the Exchange server itself. Running Spam and Virus filtering on your Exchange server will obviously reduce performance as these applications will require CPU and Memory resources as well as other system resources. Also considering the about 60-80% of the e-mail being delivered is Spam or infected by a virus, blocking this mail at the entry point will prevent them from ever getting to the Exchange server, or needing to be processed by the Exchange server. 
7. RAID No not the bug killer! Hard drives die and for that reason RAID arrays are popular and most Exchange servers will utilize a RAID array. Exchange server performs about 3 reads for every write so RAID 1+0 will provide the best performance but if you can't afford it RAID 5 is the next best thing. Of course you should be using NTFS file system too! 
8. Exchange Best Practices Analyzer ExBPA is a free tool available from Microsoft that will run a set of checks against your Exchange server(s) to determine errors in the configuration and make suggestions on ways to improve Exchange systems. It is updated often with the latest findings and improvements discovered by the Exchange team. Exchange Server Best Practices Analyzer Download 
9. Global Catalog Placement There should be at least one GC in any site that contains an Exchange server. 
10. Use the Performance Monitor Wizard Monitoring Exchange server can be complicated. There are literally hundreds of objects to monitor on the server and you probably don't know them all! Luckily Microsoft makes another free tool to automatically configure performance monitoring for your Exchange server. Performance Monitor Wizard Download