What does "Reverse DNS FAILED! This is a problem" mean?

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 is much more likely that the message is legitimate and, therefore, will be passed on to the recipient. If the IP address does not match, it is much more likely that the sending address was spoofed and, therefore, much more likely that it is unwanted and could be considered spam.

A Fully Qualified Domain Name (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 of name@mydomain.com, your PTR record should contain something like mailserver.mydomain.com). Only the organization that 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 essentially useless since no one is asking your servers.

To make any changes to your rDNS, you will need to contact your ISP. Or, if you host your own DNS (rare), you will adjust it yourself. You will not be able to do this in your DNS control panel unless your ISP also hosts your DNS and gives you the functionality to add your own rDNS records.