I subscribe to "Google Data" mailing list once. I received a few messages from this ML. I decided to unsubscribe by send an email to email@example.com (I check the email I used to send and receive). I found the unsubscribe email in the header of the mails I received and as well on the web site I used to subscribe.
But it's always ignored. I don't even have a unsubscribe acknowledgement (if it exists).