So I recently managed to make my Self-Hosted mailserver an Open Relay. This is bad.

Just because you can self-host smtp doesn’t mean you should.

I’ve run corporate email on prem for years, for tens of thousands of users, and I have no interest in self-hosting email for myself. I’ve also migrated over a hundred thousand mailboxes to EXO, and there was a good reason for that. I personally think smtp is riskier than https.

It can be all those reasons.

Did you analyze your traffic first? If you did, you probably wouldn’t be asking this question.