In case you are at least 1% curous, email natively has no real authentication, you can send a email "on behalf" of anyone. So that led to many different conflicting standards of detecting if a email ~actually~ came from the person it claims to be from. There are basically 4 such systems:
1. Reverse DNS. Really simple, if the IP that send the email has a reverse DNS of the domain name, it's ok. Very convenient but often people want to use a different domain for a mail server, or more servers are hosted on the same domain.