Every time when the Spam button is pressed by one of your subscribers, the email provider the subscriber belongs to, will send a notification related to that event. In order to receive such notifications, you should subscribe for such notifications at the email provider.
Here you can find a list with email providers and their feedback loop subscribe process.
However, most email providers will also send the notifications at one of these 3 addresses: email@example.com, firstname.lastname@example.org and email@example.com.
In order for MailBul to process these notifications, you need to add feedback loop servers for each email address, either for the one you subscribed at the email provider, or for the 3 listed above.