IPv6 sending guidelines regarding PTR 550-5.7.1

[DeltaGolf]

I appear to have missed the Server forum and previously posted this in MailPlus...

A bounce after user sent email to domain which it seems forwards to a gmail address....

someone@a-domain.tld>: host aspmx.l.google.com[2a00:1450:400c:c00::1a] said:
550-5.7.1 [2a02:8012:64b5:0:211:32ff:febe:89fb] Our system has detected
that 550-5.7.1 this message does not meet IPv6 sending guidelines regarding
PTR 550-5.7.1 records and authentication. Please review
Prevent mail to Gmail users from being blocked or sent to spam - Gmail Help for more information 550
5.7.1 . n5-20020a05600c294500b003f60682705bsi5266687wmd.151 - gsmtp (in
reply to end of DATA command)”

So while I continue to work out the trouble, what's my trouble?

At first I thought it was DKIM, but maybe not. If its just PTR is that something I deal with on the DNS and not in MailPlus?

Happy to get my sleeves rolled up once I know where I should be pointing the oil can.

The paragraph in the link Google provide is at the bottom:

Fix IPv6 authorisation errors​

An IPv6 authorisation error could mean that the PTR record for the sending server isn't using IPv6. If you use an email service provider, confirm that they're using an IPv6 PTR record.

I don't have IPv6 but in any case where does Google want to find that PTR?

-- post merged: 29. May 2023 --

So I think this is just a DKIM issue.

I have a domain mydomain.tld where you can email [email protected]. That routes via an outside MX which redirects it to my myserver.mydomain.tld when I pick it up from MailPlus Server.

But when I reply to your mail it comes direct from MailPlus Server using myserver.mydomain.tld. I think Google wants a DKIM for myserver.mydomain.tld. There is already a DKIM for mydomain.tld. Either I can use its credentials or I need to create a new TXT record in the DNS for myserver - myserver._domainkey.mydomain.tld? And then add that to MailPlus Server?

Or I create it in MailPlus Server and add that to the DNS?

But....

The only domain I have in MailPlus Server is mydomain.tld. So any DKIMs I create are for that domain, not the myserver subdomain. I need to add the subdomain to MailPlus Server?

Which is the correct way to skin this cat?