On heavily loaded Exchange 2000 or Exchange 2003 servers in a front-end/back-end configuration, POP3 and IMAP4 clients may experience slow logons and poor overall performance. The cause for this performance degradation may be the default limits of the DSAccess cache.
The DSAccess cache reduces the need to query an Active Directory domain controller. But on servers that service thousands of users, the default limits of a 4-MB cache or a five-minute Time to Live (TTL) setting may not be enough.
You can use Windows 2000 Server's or Windows Server 2003's System Monitor to measure the effectiveness of the DSAccess cache. If you determine that the DSAccess cache isn't able to handle your server's load, you can change the default limits to improve performance by making a registry edit to the Exchange server.
However, keep in mind that changing the default limits can be complex, and it involves trade-offs. Simply increasing the size or TTL of the DSAccess cache without a thorough analysis can make matters worse.
For specific information about the registry edits to change the DSAccess cache defaults, as well as calculations you should perform to make effective changes, check out Microsoft Knowledge Base article 318228.
Note: Editing the registry is risky, so be sure you have a verified backup before making any changes.