Please refer http://msdn2.microsoft.com/en-us/library/ms186358(d=ide).aspx.
To set up SQL Server Agent Mail to use Database Mail
In Object Explorer, expand a server.
Right-click SQL Server Agent, and then click Properties.
Click Alert System.
Select Enable Mail Profile.
In the Mail system list, choose Database Mail.
In the Mail profile list, select a mail profile for Database Mail.