Message[264] An attempt was made to send an email when no email session has been established

When we configure the database email, we can successfully send the test email.
How ever when we configure to set the alerts to send out email it will fail with the error
Message[264] An attempt was made to send an email when no email session has been established
which is found @ sql server agent error logs.

Fix :

select sql server agent -> right click -> properties -> alert system ->  check enable mail profile
-->  select mail system as Database Mail. --> Mail profile. --> ok  --> restart sql server agent