Firstly, it is important to note that all newsletters should have an unsubscribe link, this link should be visible and easy to use to comply with international and local regulations but some users don't use the internal link Mailrelay provides, (because their system has other opt-out options, for example) however, in some situations, you can see on your dashboard some opt-outs even if you are using your own system.
How is this possible? Quite simply, some ISPs like Hotmail, Gmail, etc., have a system that displays an unsubscribe link generated by their own software, that directs to the address provided by the ESP in the headers of the email and sends a feedback with the request of the subscriber who don't wish to receive other emails. It is a very similar system to the one used to report spam complaints.
This is an example of the code our system will insert in the email headers:
If you don't use the Mailrelay unsubscribe link, remember to check from time to time on the panel and if you find new users, add the email addresses to your list, or activate the notification system on the Settings menu: