The e-mail address and undo button on the unsubscribe web page would help. Or maybe add a title, something like:

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  HOW TO SUBSCRIBE OR UNSUBSCRIBE
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  If your friend or colleague has forwarded this to you
  and you would like to SUBSCRIBE to our mailing list,
  click here.

  If you are the subscriber and would like to 
  unsubscribe THEIR@EMAIL.ADDRESS, click here.
  ----