I want to explain to our client where the numbers from the SPAM complaints come from.  

But I actually don't know how this gets tracked or functions.

Each email campaign report shows a bunch of different statistics. And Spam complaints is one of them:
[![Spam complaints](https://i.stack.imgur.com/4JeYO.png)](https://i.stack.imgur.com/4JeYO.png)

Is it the mail client itself that sends a message back, hey your email is spam?
It's not when a user himself marks an email as spam?

**So, how does SC know when a mail was marked as spam by its receiver?**