One of our customers asks us to send out marketing mail. We manage their subscriber list in Microsoft Dynamics CRM and use a remote desktop tool to send the mails through their network via the Sendgrid SMTP server. When they receive unsubscribe requests after another mass mailing, they pass this along to us and we manually deactivate the relevant contacts in CRM. The unsubsribe link happens through a reply mail with the subject "unsubscribe".
With a recent unsubscribe wave, we found that about a dozen of the email addresses that sent such a reply, we could not find in our system, neither when searching for the first name, the last name, the email address, the domain nor parts of any of these. This isn't just "they're in there, but with a slightly different email address", because we found and deactivated a number of those through searching for the names. This is a case of "Either they're not in the system, or they're in the system with completely different credentials."
Obviously the method we're using for unsubscription is flawed and we're currently investigating alternative methods of managing the mailing list, but the problem is that those email addresses asked to be unsubscribed, but we are unable to unsubscribe them because we cannot find their registered address. If we do not unsubscribe them, we run the risk of blacklists and complaints about spam, which will affect the reach of the mailing, our reputation and our client's reputation.
How should this situation be handled?