I never thought email was simple. I thought it was straightforward. It’s not. It doesn’t matter if you follow the RFCs, you won’t have a working email server unless you listen to what the experts say.

For example, there are no RFCs about an IP address’ reputation, but that’s a real thing. When you sign up with your ISP, they’re not giving you a brand new IP address. Someone has used it before. They might have trashed its reputation, and there’s very little you can do about that. Then your emails will probably be blocked or delivered to the spam folder.