> They use their mysterious opaque algo to determine that your e-mail is spam and you have no idea what you could do to not get in peoples junk boxes.

And that’s not hard?

Running a modern mail server, a spam filter, a self-hosted webmail, and learning just enough about DNS and networking to get mail through: this is what’s not that hard.

Figuring out how to stay whitelisted, or accept ending up in people’s filters, asking them to whitelist you while your public IP gradually gains trust, is a job.