Thanks for the reply Vijay!

Yes, I just figured it out, the email message shows like this:

--------------------
Target Name=XXXXXX
Target type=Database Instance
Host=xxxxxxxx
Occurred At=Sep 22, 2011 11:21:32 AM CDT
Message=Generic internal errors have been found in the alert log.
Metric=Generic Internal Error Status
Metric value=1
Severity=Critical
Acknowledged=No
Notification Rule Name=Database Availability and Critical States
Notification Rule Owner=SYSMAN
----------------------------------------

Then logon to GC or the server to check message details.

Thanks again!
yzhang