Are there any real-world situations where a server with one or more A records is better off not having a PTR in the reverse lookup zone?
There are few services that require a PTR record. Most services neither require nor make use of PTR records. In some cases, administrators may use PTR records to perform some validation before granting access to services. In this case, they should be aware of whether or not PTR records are available. If they are aware the PTR records should not exist for the client IPs, they may use the presence of a PTR record as a red flag.
The PTR record is managed by the IP provider, which may be an entirely different organization than your DNS provider. This can make keeping the PTR records up to date difficult. This gets even more difficult if the IP address is issued by a DHCP server. The domain administrator may be unable to create a PTR record and may not be able get the IP provider to configure a PTR record.
As the posts your referred to have indicated, SMTP servers sending to the Internet will have delivery issues if they don't have a PTR record that passes rDNS. It is not necessary for an
MX server to have a PTR record unless it also sends mail to the internet. Recent anti-spam recommendations request ISPs that implement PTR records to configure PTR so that it is obvious the server should not be sending email. Many ISPs do not providing A records corresponding these PTR records.
From a mail receiving organations view, getting the PTR and A records to agree so that rDNS validation passes indicates:
- An understanding of the standard practices laid out in the relevant RFCs.
- A reasonable level of competence.
- Co-ordination of both the IP providers DNS and the domains DNS administrator.
Conversely, not having a PTR record indicates one or more of:
- An indication the the server should not be sending email to the internet.
- A degree of incompetence by the mail server's administrator.
- That the address is not under control of the organization that should be controlling it.