Select Which Database Mail Profile is used in SQL Server 2008 R2
Since "operators" are part of SQL Server Agent, I'm guessing it'll use whatever mail profile you've configured that to use.

To review/change it, right click SQL Server Agent in Management Studio, and choose Properties. Go to the "Alert System" page, and look for the "Mail profile" drop-down menu near the top. You may also have to check "Enable mail profile", and set "Mail system" to "Database Mail" if you haven't already.