I am aware that lot of e-mail spam-filters will check the PTR record of the mail-server IP address. However, what are the exact requirements for this PTR record?
Is it enough if the PTR record is something like 2.0.192.in-addr.arpa. if the IP address of my e-mail server is 192.0.2.1? In other words is it required that IP address has at least some kind of PTR record and the format of the PTR record doesn't matter?
Or is it required that the domain name part of the A record is present in PTR record? For example if the A record of my mail-server IP address is mail.example.org and domain name part of this record is example.org, then the PTR record has to be for example hosts.example.org, mx.example.org, something.example.org, etc?
Or is it required that A record and PTR record match? For example if the A record resolves to mail.example.org, then the PTR record needs to be exactly mail.example.org as well?
Last but not least, is it required that mail-server itself announces the PTR name(for example with SMTP Service ready message) and this has to match with the PTR record in DNS system?