Creating instance of all classes that have certain attribut value

I have a simple custom attribute MessageDeliveryAttribute that has one string filed. Also I have several classes that mark with this attribute like:

[MessageDeliveryAttribute("MessegaToDb")]

In the app.config are settings like:

  <appSettings>
    <add key="MessageToConsole" value="true"/>
    <add key="MessageToDb" value="true"/>
    <add key="MessageToService" value="false"/>
    <add key="MessageToFile" value="false"/>
  </appSettings>

I'm trying to implement class that will read all keys with value "true" from app.config, find all classes that marked with attribute

[MessageDeliveryAttribute("value of key from app.config where value = true")]

and then create a list with instance of these classes.
Here are working code, but I need your suggestions how to improve that?

public static List<IMessageDelivery> GetMessagesDelivery()
{
    var instances = new List<IMessageDelivery>();

    // list of settings from app.config where value is true
    List<string> settingValues = ConfigurationManager.AppSettings.AllKeys
        .Where(key => ConfigurationManager.AppSettings[key].Equals("true"))
        .Select(value => value)
        .ToList();


    foreach (var types in GetTypesWithMessageDeliveryAttribute())
    {
        var attributeValue = GetAttributeValue(types);
        if (settingValues.IndexOf(attributeValue) >= 0)
        {
            instances.Add(Activator.CreateInstance(types) as IMessageDelivery); 
        }                 
    }

    return instances;
}
// Get all classes with my custom attribute
public static IEnumerable<Type> GetTypesWithMessageDeliveryAttribute()
{
    var assembly = Assembly.GetExecutingAssembly();
    return from type in assembly.GetTypes()
           where type.IsDefined(typeof(MessageDeliveryAttribute), false)
           select type;
}
// get value of attribute
public static string GetAttributeValue(Type t)
{
    var messageDeliveryAttribute = (MessageDeliveryAttribute)Attribute.GetCustomAttribute(t, typeof(MessageDeliveryAttribute));

    return messageDeliveryAttribute != null ? messageDeliveryAttribute.DeliveryPointName : string.Empty;
}