title: List-unsubscribe in e-mail header. How-to?
body: <p>I'm trying to add a List-Unsubscribe header to my e-mail that is being sent. So far I hadn't any luck trying to do so.</p>
  
  <p>What I have got so far:</p>
  
  <pre><code>var mailMessage = new MailMessage
                      {
                          Subject = newsletter.Subject,
                          Body = newsLetterHTML,
                          IsBodyHtml = true,
                          Sender = new MailAddress(senderAddress)
                      };
                      mailMessage.To.Add(subscriber.Email);
                      mailMessage.ReplyToList.Add(senderAddress);
                      mailMessage.Headers.Add("List-Unsubscribe", unSubscribeUrl);
  </code></pre>
  
  <p>The <code>unSubscribeUrl</code> is something like 'www.example.com/unlist?id=8822772727'.
  When I sent the e-mail everything works fine except for the list-unsubscribe option. Which is not shown in any mail client.</p>
  
  <p>Any assistance would be welcome!  </p>
  
  <p><strong>UPDATE</strong><br>
  This is the whole code I use for sending the email:  </p>
  
  <pre><code>var mailMessage = new MailMessage
                      {
                          Subject = newsLetter.Subject,
                          Body = newsLetterHTML,
                          IsBodyHtml = true,
                          Sender = new MailAddress(senderAddress)
                      };
                      mailMessage.To.Add(subscriber.Email);
                      mailMessage.ReplyToList.Add(senderAddress);
                      mailMessage.Headers.Add("List-Unsubscribe", String.Format("&lt;{0}&gt;", "http://www.foo.com/unlist?id=8888"));
                      mailMessage.HeadersEncoding = Encoding.Default;
  
                      var smtpClient = new SmtpClient();
                      smtpClient.Send(mailMessage);
  </code></pre>
  
  <p><strong>UPDATE 2</strong><br>
  After a little research I got the header into the mailMessage. When I sent an email I can see the following headers:    </p>
  
  <pre><code>List-Unsubscribe: &lt;http://demo.localhost/home/hobbymap-gerard/afmelden-voor-nieuwsbrief?id=c786aeb0-554d-4670-94d8-82d6f25a050b&gt;
  MIME-Version: 1.0
  From: info@test.nl
  To: test@gmail.com
  Reply-To: info@test.nl
  Date: 8 Feb 2011 09:50:22 +0100
  Subject: Test met plaatje
  Content-Type: text/html; charset=us-ascii
  Content-Transfer-Encoding: quoted-printable  
  </code></pre>
  
  <p>But when I open the email in any client I can't see the unsubscribe button in the client? Am I doing something else wrong?</p>