Delivery Notification through C# 1.1

I want to receive a delivery notification when sending an email through ASP.NET 1.1 C#.

I can get a read notification using;
testMessage.Headers.Add("disposition-notification-to", txtFrom.Text);

However I cannot get a delivery notification. I have tried;
testMessage.Headers.Add("Return-Receipt-To", txtFrom.Text);
and/or
testMessage.Fields.Add("urn:schemas:mailheader:return-receipt-to", txtFrom.Text);

Neither work.

I am sending from domain 1 to domain 2. They are not linked in anyway. Manually (through outlook) sending an email with request delivery notification selected works perfectly from domain 1 to domain 2. However when I create and send the same email through a windows/ASP.NET application no delivery notification is returned.

I know this is made much simpler in .Net 2.0+. But that isn't an option with this application. Does anyone have a solution to this issue?

System.Web.Mail.MailMessage testMessage = new System.Web.Mail.MailMessage();
				System.Web.Mail.SmtpMail.SmtpServer = txtSMTP.Text;
			
				testMessage.To = txtTo.Text;
				testMessage.From = txtFrom.Text;
				testMessage.Subject = txtSubject.Text;
				testMessage.Body = txtBody.Text;
				testMessage.BodyFormat = System.Web.Mail.MailFormat.Text;
				if(chkDelivery.Checked)
				{//Not Working! :-(
					testMessage.Headers.Add("Return-Receipt-To", txtFrom.Text);
					//testMessage.Fields.Add("urn:schemas:mailheader:return-receipt-to", txtFrom.Text);
				}
				if(chkRead.Checked)
				{
					testMessage.Headers.Add("disposition-notification-to", txtFrom.Text);
				}
				System.Web.Mail.SmtpMail.Send(testMessage);

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