In many news letters I receive in html format when you see in text format there is a message at the beginning that states: "If you can not see the message correctly open the following link: <newsletter URL>". Users do not always read their mail in the same applications, so sometimes the choose the more fancy newsletter and once in a while hey have to read it in an non html application.

Could you create a new parameter, like the #unsubscribe# parameter, that has the URL of the newsletter archive?