We are a small ISP, using linux DNS servers. Recently a client has asked us to create a rDNS zone for a specific IP address, which is used for a mail server. They keep getting message failures for some emails due to lack of PTR records and valid rDNS.
We created the rDNS zone the same way as all of the other rDNS zones are created. The only difference is that, outside of our network the PTR record is not found, but only for that specific IP address.
Does anyone know what could be the cause of this?