Reverse DNS (PTR) for your mail servers
Reverse DNS maps an IP address back to a hostname through a PTR record in the in-addr.arpa (IPv4) or ip6.arpa (IPv6) zone. For mail, the IP a server sends from should have a PTR that resolves to a name, and ideally that name resolves back to the same IP (matching forward and reverse, "FCrDNS").
Why it matters
Many receiving mail systems check the sending IP's PTR before accepting a message. A missing PTR, or a generic one that looks like a dynamic/residential address, is a common reason legitimate mail is delayed, filtered, or rejected. It is one of the cheapest deliverability wins.
Whose job is it
PTR records live with whoever controls the IP address - usually your hosting provider or the operator of the network the mail server sits on - not with your domain's DNS. You typically set it through the host's control panel or a support request, not your zone editor.
What the report checks
That each MX target's IP address has a PTR record (a reverse lookup returns a name). A missing PTR is a warning, since it hurts deliverability without being a hard misconfiguration in your own zone.
How to fix it
Ask your mail server's host (or set in their panel) a PTR for each sending IP that points at a real hostname you control, for example mail.example.com, and make sure that hostname also has a forward A/AAAA back to the same IP.
How to verify
dig -x 203.0.113.25 +short # IPv4 PTR
dig -x 2001:db8::25 +short # IPv6 PTR
The name it returns should also resolve forward to the same address.