how to troubleshoot MSExchange ADAccess issue

NLTest /DSGetSite

2601, 2604, and 2501 MSExchange ADAccess Event IDs when a Microsoft Exchange server restarts

http://support.microsoft.com/kb/2025528

Overview of the Microsoft Exchange Server Analyzer Tool

http://technet.microsoft.com/en-us/library/bb508838(EXCHG.80).aspx

Microsoft Exchange Server Analyzer - Articles

http://technet.microsoft.com/en-us/library/aa997090(EXCHG.80).aspx

Microsoft Exchange Analyzers

http://technet.microsoft.com/en-us/exchange/bb288481.aspx