We have an opt-in service that people leave via a list-unsubscribe request. We attempt to honor all such requests immediately.
Unfortunately, it is all too common that we receive a list-unsubscribe request from an email address that is not in our list. We believe this happens when users have set up forwarding and are responding from the forwarded address. We try to go the extra mile to identify such users, because sometimes you can identify the user due to similarity to another email address that is in our system.
But in most cases the user cannot be identified and we have no way to contact them other than sending an email to the provided address explaining that the requested removal cannot be accomplished, and that they should let us know other email addresses they may have used. This of course has the ironic consequence of sending them more email! Normally such a request from us is unanswered.
There is a four-year old question on this topic at what should I do if I cannot find a person who asks to unsubscribe in my mailing list?, but the answers there are not fully applicable. We do have an "unsubscribe" in our footers, but this question pertains to people who do not use that, but rather use the list-unsubscribe function of popular email services.
I tried to find any discussion of best practices in this area, but did not find anything. Anyone solve this one?