They require that
a) the sending IP address has a PTR record b) from a domain that you own c) that resolves to the same IP address.
This is also very important for general deliverability (https://en.wikipedia.org/wiki/Forward-confirmed_reverse_DNS).
They furthermore recommend d) that your host name should clearly mark it as a mail server and e) to make sure the domain leads to a web page that contains provider details and contact information.