Ben over at MailChimp writes about spamfilters that are following links in emails resulting in people being unsubscribed from lists without their knowledge. I strongly suggest clients use a 2 step unsubscribe system, that does not require any passwords or information. The recipient clicks on a link in the email and confirms that they do want to be unsubscribed once they get to the unsubscribe webpage.
Even more concerning for me is the idea that people could be subscribed to emails without their knowledge. For some subset of lists, using confirmed (double) opt-in is the best way to make sure that the sender really has permission from the recipient. Now we have a spam filter that is rendering “click here to opt-in” completely useless. I am sure there are ways to compensate for the stupidity of filters. As usual, though, the spammers are doing things which push more work off onto the end user and the legitimate mailers.