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?
We do that upstream, no way for you to avoid it. For good reason too, our team handling abuse notifications mails was super swamped with people whose ancient XP PCs had malware sending spam.
Forget running your mail server on a residential IP anyway. You’ll be instant blocked by any mail provider, residential IPs are always spam, because of the aforementioned XP PCs.
Personally I wouldn’t self host mail anymore anyway. Too much trouble.