When a sending server makes a connection to the recipient server, the recipient server notes the sending IP address and performs a reverse lookup. This is done by sending a DNS query which returns a Fully Qualified Domain Name ( FQDN) registered for that IP address. If the sending SMTP address matches the domain, then it’s much more likely that the message is legitimate and therefore will be passed on to the recipient. If the IP address doesn’t match, it’s much more likely that the sending address was spoofed and therefore much more likely that it’s unwanted and could be considered spam.
A FQDN is associated to an IP with a valid PTR record. You want the domain name portion of the FQDN to match the domain of your email address. (e.g. if your sending addresses follow the convention email@example.com, your PTR record should contain something like mailserver.mydomain.com). Only the organiztion which controls and owns the IP can set a PTR record. PTR record queries are sent to the owner of the IP address which is the ISP, unlike other DNS queries which are sent to the DNS server of whoever owns the domain. For this reason Setting a PTR record on your own DNS servers is almost useless since no one is asking your servers.