I managed to fix this , but got into another problem.
1. I added a message in sys.messages by executing a stored procedure
EXEC sp_addmessage @msgnum = 50603,
@severity = 16,
@msgtext = 'Invalid number'
2. Then used this messageid to raise error in the stored procedure .
like RAISE ERROR(50603,16,1) WITH lOG - i initially had only RAISE ERROR (50603,16,1) it didnt work. I changed that to RAISEERROR(50603, 16, 1) WITH LOG and it worked. All severity levels less than 19 should have WITH LOG or use xp_logevent.
After these two changes the alerts worked. However i was getting continously alerts for long time more than 20 alerts until I volunterly changed the stored procedure back to RAISE ERROR(50603,16,1) which will not alert.
I am wondering why I am getting continous alerts though I was not executing the job. I just executed the store procedure from query analyser to raise the error..
Any help on this??