If you have moren than 1 GB of memory in your Exchange Server do the following settings. These settings are not recommended on a front-end server, dedicated bridgehead or when you have Exchange installed on a Domain Controller.
Add the switches /3GB and /USERVA=3030 to boot.ini. The /3GB switch modifies the way virtual address space is created so that 3 gigabytes are available for user mode applications
Configure the following registry key:
Value = 0x00040000 (HEX).
The HeapDecommitFreeBlockThreshold registry key specifies the number of contiguous bytes above which the memory is decomitted rather than retained for reuse, thus avoiding virtual memory fragmentation.
Verify the following key
HKLMSYSTEMCurrentControlSetControlSession ManagerMemory ManagementSystemPages
The value must be set to 0. If you have a server with more than 2 GB of memory, it may help to increase the size of the Store Database Cache (aka ESE buffer).
For more information: How to optimize memory usage in Exchange Server 2003