The alert for 'time behind' has been raised. The current value of 'xxx' surpasses the threshold 'xxx'.
Usually the above error will report only when there is unsent log has stuck up in the principal server.
This can be cause when there is heavy load in the principal server or if there is any network slowness between principal and mirror server and couple of other things.
I encountered the above error on the principal server and error number is 32040 is detected in the event viewer,
First if you want to identify which database is not in sync, to do that you need to launch database mirroring monitor wizard and add all the databases and verify the database which has unsent log
Second Launch mirroring monitor and add the problematic database and verify the sync status with the mirror server.
If you dont find any unsent log and unsent log is showing zero (0) kb, then I used Pause and Restart mirroring to fix the issue.
Alter Database KalyanDB Set Partner Suspend -- It pauses the mirroring on Kalyandb
Alter Database KalyanDB Set Partner Resume -- It resumes the mirroring on Kalyandb
If you want to do it from SSMS, you can do that by using the database properties window.