List-unsubscribe in e-mail header. How-to?

Question

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.

What I have got so far:

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);

The unSubscribeUrl 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.

Any assistance would be welcome!

UPDATE
This is the whole code I use for sending the email:

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("<{0}>", "http://www.foo.com/unlist?id=8888"));
                    mailMessage.HeadersEncoding = Encoding.Default;

                    var smtpClient = new SmtpClient();
                    smtpClient.Send(mailMessage);

UPDATE 2
After a little research I got the header into the mailMessage. When I sent an email I can see the following headers:

List-Unsubscribe: <http://demo.localhost/home/hobbymap-gerard/afmelden-voor-nieuwsbrief?id=c786aeb0-554d-4670-94d8-82d6f25a050b>
MIME-Version: 1.0
From: [email protected]
To: [email protected]
Reply-To: [email protected]
Date: 8 Feb 2011 09:50:22 +0100
Subject: Test met plaatje
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: quoted-printable  

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?

Answer 1

Most email clients only support mailto-links.

Try this, it should work always:

mailMessage.Headers.Add("List-Unsubscribe", "<mailto:[email protected]?subject=unsubscribe>");

The RFC specifies it is possible to use http-links also. But i've found that if you include http-links, the email clients no longer shows the unsubscribe link. I think it's because of the possibility that people only have access to the mail protocol.

So this does not always work:

 mailMessage.Headers.Add("List-Unsubscribe", "<http://www.host.com/list.cgi?cmd=unsub&lst=list>, <mailto:[email protected]?subject=unsubscribe>";