That’s not what the error message is saying. The error message;
Is saying that Google has accepted an inbound connection from your IPV6 adddress, and the in-addr for that IPV6 address doesn’t match what it’s looking for. You can either disable IPV6, or fix the IPV6 PTR record for 2a01:4f8:161:43aa::2
I think the easiest way would probably be to go to webmin, networking, network configuration, then in the right hand window select your Ethernet interface, click on the radio button for “IPV6 Disabled” and restart networking/reboot the server. I’m sure others can provide a better/different way. I’ve not personally experienced anything in my environment that would be helped by using IPV6.
Yeah, in the second screenshot I would make that say “IP6 Disabled” - but I’m not sure if there’s a dependency for IPV6 in your environment that does not exist in mine. It’s entirely possible, if not likely, that your virtual servers have also used an IPV6 address and won’t restart properly if you disable IPV6 here. It would take some close examination of your environment to see if disabling this is entirely safe.
The other two ways you can fix this would be what was suggested above; disable IPV6 in postfix with the flag (you’d have to edit the postfix config file by hand I think, I can’t find a way offhand in the UI), or see who has the PTR records for your IPV6 address and have them update it.
I’d like to point out a common misunderstanding regarding the PTR (reverse DNS) record: Whether it is IPv4 or IPv6, the PTR record is controlled by the entity controlling the IP.
You can simply ask the service provider from which you were assigned this IP to add the PTR record for you, if they don’t already offer some interface for you to do it yourself…
PTR has nothing to do with your domain or DNS settings. It is stored in a separate location, but if you want it to be useful for email, you should use a valid resolvable host name, such as mail.example.com.