Your mail server works. Postfix starts, the logs look clean, you send a test message to your Gmail — and it lands in spam. Or it just disappears. Nothing in your config is broken. The software did exactly what you told it to.
The problem is that the server on the other end doesn't trust your IP yet, and email hides a whole stack of small trust checks that all have to line up before a stranger's inbox lets you in. Get those right and delivery mostly takes care of itself. Miss one and you're shouting into a spam folder.
Three things carry most of the weight: reverse DNS, sender authentication, and whether your host even lets you talk on port 25. None of them are hard. They're just easy to forget, and each one is a quiet veto.
The one everyone forgets: reverse DNS
Forward DNS is the part you know — a name points to an IP. Reverse DNS is the mirror: an IP points back to a name. That record is called a PTR, and it lives with whoever controls the IP, not in your domain's zone.
