I got this error since today aswell. I emailed with my friend on gmail everyday, yesterday was fine too, but today this came up:
host gmail-smtp-in.l.google.com[ipv6xxxxx]
said: 550-5.7.25 [ipv6xxxxx] The IP address sending this message
does not 550-5.7.25 have a PTR record setup, or the corresponding forward
DNS entry does 550-5.7.25 not match the sending IP. As a policy, Gmail does
not accept 550-5.7.25 messages from IPs with missing PTR records. For more
information, go 550-5.7.25 to 550-5.7.25
I do have PTR correctly set. My mail-tester is 10/10. No changes this week.
edit: I have 2 vps with same configurations and aapanel versions. One of them have error as above, the other is fine.
The one with IPv6 has this problem. The one with Ipv4 is fine. So i dont think it is problem with ipv4 or ipv6...