SES Complaints - how does a user complain?

Let me preface this by saying that I run a very small service with less than 50 active customers. It is a GPS tracking service where the customer decides on the email alerts they will receive about their tracker(s).

I've started receiving complaint emails for one of my customers, so I contacted them about it. They said that they did not intend to complain and that they value the emails they receive and must have accidentally done this on their phone somehow. However, I have received another 4 complaint emails over the last 24 hours. So how does the user actually complain, and how can this be done by accident?

My emails always explain why they are receiving it, and also provide instructions/link on how to configure the alert emails they receive.