No record in directory service in event viewer on mail server.

I am a system administrator new to a small size company. On my mail server, when i tried to check the event logs on that server, after i open Event Viewer, I found there is no record under directory service node, on the contrary to this, there are 748 events in applciation log within two days (Sep 30th and Oct 1st), 153 events log in security log, and 2684 events in system log, and 1569 events in file replication service.

What causes the directory service events to be zero? I think something wrong around in my domain, so it doesn't catch the mistakes. Is there anyone here can help me with, thank you very much.