If you have developer experience, there are two ways you can capture unsubscribes in your own system.
Webhooks
A webhook is a user-defined callback in the form of an HTTP POST, which is triggered when something changes.
Custom unsubscribe confirmation pages
Every list can have its own unsubscribe confirmation page, which subscribers see after unsubscribing via an email link or unsubscribe form.
To record the email addresses of unsubscribers and other information, you can replace our generic unsubscribe confirmation page with your own by pointing to a new URL, and including a query string that contains custom fields. For example:
www.example.com/unsubscribe.php?user=[email]In the URL above, [email] will be swapped out with the unsubscriber's email address when they use an unsubscribe link, unsubscribe form, or the "Unsubscribe" button in their subscriber preference centre. Your own code can then retrieve that address, along with the information stored in any other custom fields you include in the URL.
To test that the custom fields are passed through to your page correctly, you will need to send a campaign — quick test emails will not replace custom field tags.