Today, I received an email from the development team complaining why they have not received any of the SQL Agent job notification emails today. Upon checking the SQL Agent error log, I see the following error message:

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

To investigate the issue, I followed the steps below to verify that SQL Server Agent Alert System is configured correctly and is associated with the correct Database Mail profile:

• Right Click on SQL Server Agent, Select properties.
• Select Alert System.
• Select Enable mail profile.
• Select OK.

I also successfully sent a test email using this Database Mail profile.

Although, I verified above settings and sent test emails using Database Mail, SQL Server Agent Job was still unable to notify the SQL Server Agent Operator. At this point, I decided to Google this error message because I was not sure why this error is occurring. After digging around online, I realized I just need to restart the SQL Server Agent service. After I restarted SQL Server Agent service, SQL Server Agent job notifications started working again.