[pepa65]

Running an smtp server to receive mail is supereasy as long as you have inbound port 25.

Or you can run a submission service that requires submitters to login, usually on port 587/465.

If you want to send from your server, that is way more difficult, requiring all kinds of safeguards, SPF, DKIM, ARC, reputation, etc. They keep making it more complicated, because that's the source of spam.

Or you can just submit mails to a relay, that will send mails for you, this can even be Google or some other MX service. This then always requires you to authenticate with your account.