Creating an Email Notification based on a dropdown field in your ConvertForm is really simple with the use of a Smart Tag.
First, we need to create the dropdown field with the options of it being actual emails as shown in the screenshot below.
Second, we need to create the email notification and in the "Send to Email Address" field with the help of the Smart Tag {field.FIELD_NAME} we can set it to use the dropdown field we created earlier.
As you can see for the purpose of our example we used the Smart Tag {field.recipient} because we named the dropdown field as "recipient" in our first step.