In your mail service program, the mail failure can be captured using exception
MailAddress from, to;
try
{
MailMessage mailMsg = new MailMessage();
.........
SmtpClient smtpClient = new SmtpClient("mailserver", port);
smtpClient.Send(mailMsg);
}
catch (IOException ex)
{
MailMessage mailMsg = new MailMessage();
to = from;
.........
smtpClient.Send(mailMsg);
}
Apparently, the bouncing mail notification can reach the originator.