Secret Read Receipt Software Needed

I am looking for a program that will give me a "secret" read receipt when my emails are read. Being able to designate individual emails for this service would be nice. We use IE8's Request Read Receipt but many clients, especially those currently paying slow, don't elect to return the read receipt. They then deny receiving the past due notice. I remember having one of these with a past service provider "Xpress America" and it worked great! This is the site that is going to make my day! Any ideas?