Add a List-Unsubscribe e-mail address header to campaigns. Gmail uses it to automatically unsubscribe users who click an unsubscdribe button in their own webmail, and it improves message delivery rates by lowering spam scores! It is win-win.
A List-Unsubscribe header might look like this:
From: josh@domain.com
Subject: [espc-tech] More info on List-Unsubscribe
Date: August 31, 2006 3:13:02 PM CDT
To: jasmine@domain.com
List-Unsubscribe: <mailto:unsubscribe-espc-tech-12345N@domain.com>, <http://domain.com/member/unsubscribe/?listname=espc-tech@domain.com?id=12345N>As you can see, it supports both an email based (mailto) and web based (http) unsubscribe mechanism.