c# enum exclude

Let's say I have an enum like this:

[Flags]
public enum NotificationMethodType {
    Email = 1,
    Fax = 2,
    Sms = 4
}

And let's say I have a variable defined as:

NotificationMethodType types = (NotificationMethodType.Email | NotificationMethodType.Fax)

How do I figure out all of the NotificationMethodType values that are not defined in the "types" variable? In other words:

NotificationMethodType notAssigned = NotificationMethodType <that are not> types