Overview
There is no 100% guarantee. However, there are some good clues.
Mailgun provides DKIM and SPF verification for incoming mail, which is shown in the MIME headers once spam filtering is enabled in the Control Panel. This way you can at least know if the message is coming from an authenticated server.
Need Support?
Our Support Team here at Sinch Mailgun is happy to help! Reach out to us in the Support page of your Mailgun Control Panel, and we'll be with you shortly!