Own server mail service for newsletter

After no one answering my last post about enabling DKIM in the mail, I’ve did it and my mail is both, sent using TLS connection and signed with DKIM. But the issue is, I can’t send newsletter using it since Ghost keeps asking for Mailgun settings.
Now I’m not sure, is ghost being supported by them, or ghost team just LOVE that provider?

How can I force Ghost to send email newsletter using my own server?
I’m saying force to get help about getting around the limitation, not to say I didn’t find the button to click it.

Thank you!

Ghost does not support other mail services at present, which is clearly outlined in the docs, as well as in multiple other forum topics. Please read the docs before posting here.