You will need to do this as a HTML message stamp, as plain text will not provide the functionality that you are looking for. I would start by wrapping the whole thing in paragraph tags so that it is displayed properly. The email link can be just a normal mailto anchor tag.
<p>If this email you received is Spam mail please click here: <a href="mailto:email@example.com?Subject=Spam%20message">firstname.lastname@example.org</a></p>
Note that this will have the user compose a new email message, but with the to and subject fields pre-populated. This will not include any information about the alleged spam message, and there is no way to include that.