How to maintain/preserve Custom Email Headers(X-header) when replying

i am using following code to send an email from my .NET application ,here i have added custom header X-Company. When the email is received i get this custom header properly but whenever i reply/forward this mail Custom Headers not getting back.

MailMessage message = new MailMessage(From, To);                  
                message.Headers.Add("X-Company","xyz");                
                message.Body = emailMessage.ToString();
                message.Subject = Subject;                
                message.IsBodyHtml = true;               
                smtp.Send(message);