SQL server Agent Job email notifications not being sent

I trying to set up some failure job Email notifications on Sql Server Agent jobs. 
So, I configured database mail and tested and was able to receive mail through sp_send_dbmail and "Test mail"
option under database mail.
 
I enabled the mail session on sql server Agent and selected DatabaseMail and <<ProfileName>> for Mail system and Mail Profile
options and restarted the sql server agent. But still I do not receive email notifications. 
also, the operators are configured fine and have correct email id assigned.

below is what I see in the agent error log after I restarting
Message

[355] The mail system failed to initialize; check configuration settings
[264] An attempt was made to send an email when no email session has been established

Do not know why it says no email session has been establised. SQL Server Version : Sql server 2005 standard edition SP3(x64).
Also, I restarted Sql server agent several times. so, I think that's not an issue.

again, I can receive mails when I do sp_send_dbmail but it is only issue sql server agent mail notifications.

Please advice.

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Hope it Helps!!