I have been trying to tell people that this is happening for years. More about that spoofers were doing it, not what tools they were using

I didn’t know I needed proof because people often resorted to victim blaming even though I never fell for any of the emails

I think it's difficult to prove, and the only people who care will be the domains owners that are impacted. If you send enough mail, you won't notice a few extra emails going out from another account.

Mailgun should have a way to monitor and block this. We used SMTP to interface with Mailgun and frankly, I didn't even think of this as being a vulnerability until I left the service. The DMARC reports just prove it was happening.

Josh/CTO of Mailgun here - I took a look at this case and these messages were being sent from the sandbox feature of our service. The sandbox only allows messages to be sent to "authorized recipients" through an opt-in mechanism, which makes it impractical for spamming and phishing. The purpose of this feature is to allow customers to get comfortable with the interfaces and test the product in a safe way without having to add DKIM/SPF records. I reviewed these messages and confirmed there is no illicit behavior, likely just a misconfiguration from a user of radiotoolbox.

I know it's accurate, I had another domain on my machine that was sending email through my account, on accident, and Mailgun did nothing to stop it.

Judging by the reputation of the new Mailgun owner, I wouldn’t be surprised to find out this is a feature for scammers, not a bug.