Writing this because there seems to be no correct documentation on the relevant google websites and it turns out to be non-trivial. Our goal here is to unsubscribe from a ordinary google group.
Mails from the google group contain the quoted footer:
--
You received this message because you are subscribed to the Google
Groups "FOO" group.
To unsubscribe from this group and stop receiving emails from it, send
an email to FOO+unsubscribe@googlegroups.com.
Visit this group at http://groups.google.com/group/FOO
For more options, visit https://groups.google.com/groups/opt_out.
Seems easy enough, so let's send a Mail to this FOO+unsubscribe address. Back comes a E-Mail:
From: FOO <FOO+unsubconfirm@googlegroups.com>
Subject: Unsubscribe request for FOO [{EJzZjpgFhDHd9seTdRA0}]
To: Christoph Egger <christoph@example.com>
Date: Tue, 18 Feb 2014 18:55:24 +0000 (38 minutes, 53 seconds ago)
[Leave This Group]
Visit Go
[Start] your own group, [visit] the help center, or [report]
abuse.
So click on the [Leave This Group] link and be done? Unfortunately not. Looking at the link you notice it's called http://groups.google.com/group/FOO/subscribe -- no token and "subscribe"? I actually want to unsubscribe! And indeed, clicking gets an Interface that offers to "Enter the email address to subscribe:" + Captcha. And whatever it does, it -- of course -- doesn't unsubscribe. (My guess is, it would actually work if you had a real google account associated with that email address and were logged in to that account but there's no way of verifying this as already the first condition is false in this case)
Now if you disable HTML completely for the email, a totally different content emerges:
Hello christoph@example.com,
We have received your request to unsubscribe from FOO. In order for us to complete the request, please reply to this email or visit the following confirmation URL:
http://groups.google.com/group/FOO/subscribe
If you have questions related to this or any other Google Group, visit the Help Center at http://groups.google.com/support/.
Thanks,
Google Groups
Still the non-functional link, however it also mentions a different solution: "please reply to this email" which was not present in the HTML mail at all. And it works.