To send a notification in response to an alert, you must first configure SQL Server Agent to send mail.
Using SQL Server Management Studio; to configure SQL Server Agent to use Database Mail:
In Object Explorer, expand a SQL Server instance.
Right-click SQL Server Agent, and then click Properties.
Click Alert System.
Select Enable Mail Profile.
In the Mail system list, select Database Mail.
In the Mail profile list, select a mail profile for Database Mail.
Restart SQL Server Agent.
Note: Prerequisites include:
Enable Database Mail.
Create a Database Mail account for the SQL Server Agent service account to use.
Create a Database Mail profile for the SQL Server Agent service account to use and add the user to the DatabaseMailUserRole in the msdb database.
Set the profile as the default profile for the msdb database.
Reference:
https://docs.microsoft.com/en-us/sql/relational-databases/database-mail/configure-sql-server-agent-mail-to-use-database-mail