It creates a cost for spammers. They have to have an account with a Telco, which isn’t free, which in a lot of countries comes with some sort of National ID to register. That’s the reason.

I pay $6USD/month for a cloud box that’s encrypted and has been running for years, no outages, no downtime.

For a large organization? Sure. For your family and friends? No.

There are lots of tutorials, and choosing a security-first OS is also wise… OpenBSD is my current favourite.

Honestly, building a mail server isn’t that hard if you aren’t afraid of the command line.