Read the linked article. Email clients wrap the contents of forwarded messages in a <blockquote>, you just have to use CSS rules to hide the contents of a "blockquote yourfooterclasss" while otherwise showing yourfooterclass by default.

EDIT I actually meant "read the article linked from the linked article" — I too spent a good minute there trying to find out their solution.

Read here: http://jonathan-kim.com/2011/programmatically-customizing-a-...

Basically most e-mail clients change the dom structure in some way when you forward an email (like wrapping the original in a div). Write some CSS rules that require an unmodified structure to show the unsubscribe button.