These days it's hard to make a mail server an open relay. By default they are no longer open. It's been like that for the last 10 years or so.
The defaults are safe, the various popular guides you'll find on how to setup something like Postfix with Dovecot will result in a safe setup and if you use something like sovereign ( https://github.com/sovereign/sovereign ), you'll end up with a very well configured mail server with very little work.
The defaults are safe, the various popular guides you'll find on how to setup something like Postfix with Dovecot will result in a safe setup and if you use something like sovereign ( https://github.com/sovereign/sovereign ), you'll end up with a very well configured mail server with very little work.