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

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 =开发者_C百科 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: 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  

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?

Most email clients only support mailto-links.

Try this, it should work always:

mailMessage.Headers.Add("List-Unsubscribe", "<mailto:list@host.com?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:list-request@host.com?subject=unsubscribe>"; 

According to the List-Unsubscribe website, the URL should be wrapped with angle brackets, e.g. <https://www.example.com/unlist?id=8822772727>.

You can try something like:

mailMessage.Headers.Add("List-Unsubscribe", String.Format(
    CultureInfo.InvariantCulture, "<https://{0}>", unSubscribeUrl));

To ensure you are not flagged as spam make sure to have an SSL Certified domain.

In addition to other answers, there is also RFC-8058 that requires another header to enable HTTPS link unsubscribe functionality:

List-Unsubscribe:<https://example.com/unsubscribe.html?opaque=123456789>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

List-Unsubscribe-Post header should have a value List-Unsubscribe=One-Click. This is to prevent accidental unsubscribe by anti-spam software and allow an extra step of displaying a web page with an unsubscribe button.

Some email client will not process List-Unsubscribe links without List-Unsubscribe-Post header.