This is a result of the fall back unsubscribe form.  This form exists as a plan B for if there is a problem with the unsubscribe page.

The most common cause of this is the unsubscribe link being broken. An unsubscribe link should look like this:

http://customer.convio.com/site/CO?i=s-mQ4iueeYhzs9FGEbYl8Ye5_FoZV6wd

If the URL is broken you will be taken back the fall back unsubscribe form.  Examples:

http://customer.convio.com/site/CO?i=s-mQ4iueeYhzs9xxxxxFGEbYl8Ye5_FoZV6wd
http://customer.convio.com/site/CO?i=brokenurl
http://customer.convio.com/site/CO?i=s-mQ4iueeYhzs9
FGEbYl8Ye5_FoZV6wd

The second most common cause of the constituent receiving this page is if they have cookies disabled.  This is fairly rare though as most people leave cookies enabled.

What should you do when you see these?
Unsubscribe the constituent by hand on the backend.

What should you do if you see a sudden spike in such emails?
Typically this is simply a sign that you have sent out a really large email.  However, if you send large emails regularly and you are suddenly receiving a large volume of these unsubscribe emails, there may be a problem with the email you recently sent breaking URLs.  There is nothing you can do to fix the already sent email but you may want to take care to not use a direct copy of that email again so as to avoid having this problem again.

What if the constituent requesting to be opted out is not in the system?
Spam users can also trigger these messages to send by filling out a form on your site.  If you determine a user is submitting this over and over with the same information then you may want to set up a rule in your Outlook to handle emails with certain content in the body differently.  The form is a templated page in Luminate Online and at this time there is not a place to edit how it works or functions.