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 of email providers and their feedback loop subscribe process.
However, most email providers will also send the notifications at one of these 3 addresses:
firstname.lastname@example.org, email@example.com and firstname.lastname@example.org.
In order for Easy-mails 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.
Please note that when sending only via delivery servers of type web api you don’t need to setup feedback loop servers since these are automatically processed via webhooks.