Interesting topic on Technet
Source: http://technet.microsoft.com/en-us/library/cc671175(EXCHG.80).aspx
This topic provides guidance about the most useful performance counters to monitor that are common among all Microsoft Exchange Server 2007 server roles. When monitoring Exchange 2007 servers, you should know which performance aspects are most important. The common counters and threshold values detailed in this topic can be used to proactively identify potential issues and help identify the root cause of issues when troubleshooting.
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