I’m aware most ISPs do not allow for port 25 to be open for email use outside of business licenses, but at what level is that controlled? Can I get around that by owning my own router? Owning my own modem or ONT? Or is this just a thing they mystically control further up the pipeline that a relative layman such as myself can’t get around?
I’ve been running my own mail server using Mail-in-a-box on a digitalocean VPS for about 10 years. I also pay for an external SMTP relay service because I still get randomly blocked by Google/Microsoft/whatever just by virtue of having a digitalocean IP.
Total cost is $15/mo for the VPS and $50/yr for the relay service.
You’re already using an SMTP relay so why not host your Mail-in-a-box server at home? Been doing that for years. Also, check out Mailcow if you’re interested in running your server as a docker container.