I believe there is an issue with your Mailgun configuration.
I remember, in another thread you mentioned that you tried setting up transactional emails via Mailgun – so the fact that you had issues with sending emails to Gmail when using Mailgun, but it works with fast comet’s email service, points towards an issue with Mailgun for me.
This error code is thrown in context of Ghost calling on Mailgun (or, specifically the “bulk email service”, which then calls Mailgun) to send a newsletter. So, another “symptom” that points towards your Mailgun configuration, in my eyes.
I’d be interested to know whether you see any logs in Mailgun?