Show custom headers in the events api to help identify emails sent.
Sometimes mailgun will trigger a 5XX error for some emails sent through the api but still receive and send that email. I have verified this in the logs. I have a customer header I use in my emails X-MS-ID=someid I use to identify emails. I can use the search api to find and identify emails which I don't have an id for and prevent sending duplicate emails resulting in user complaints.