Diagnostics logging levels determine which Internet Mail Service events are written to the Windows NT Event Log and to the SMTP protocol log and message archival directories. They represent a progression of detail from recording only highly significant events, such as an application failure, through moderately important events, such as receipt of messages across a gateway, to events relevant only to debugging. Typically, you want to log only critical events. However, when problems occur, diagnostics logging lets you change the logging levels to capture more detailed events.
For more information about diagnostics logging, see Microsoft Exchange Server Maintenance and Troubleshooting.