Sending Email with return-path not functioning

Question

I am using System.Net.Mail email. in the code i am setting the return-path of email as follow:

string sReturnPath = ConfigurationManager.AppSettings["ReturnPath"].ToString();
if (sReturnPath.Length > 0) {
    msg.Headers.Add("Return-Path", sReturnPath);
}

If the delivery has failed it should go to return-path but it doesn't, even though I can see the header of email's return-path being from config file that I specified. The email gets returned to sender.

any ideas?