Thanks for your support answer.
on Checking with Agent Log history -- > Unable to send job completion notification email to the operator 'UserDBA' for job 'update Statistics = USER DATABASES'
Prior to this couple of weeks ago it was running fine but on only SUNDAY the completion Notification is DENIED sending to Operator.
-->Should it be understood though the COMPLETION NOTIFICATION not sent but JOB DONE
Notification to the operator is needed (Including Sundays).
Couldn't find any more clue. How to get this done please.