We had the problem that one domain (freenet.de) started replying with “550-inconsistent or no DNS PTR record for <IPv6> (see RFC 550 1912 2.1)”. Seems that they started checking whether the reverse DNS to the IP is a plausible one. (DNS PTR record and Revese DNS Entry are synonyms.)
Trying to set reverse DNS for IPv6
On one server we never even set the DNS PTR record for the IPv4 – we adjusted that. Yet the webtropia (our server hoster) interface for the reverse DNS for IPv6 did not work – no feedback, nothing. We asked the support which replied to just disable IPv6 on the server for the network adapter.
Trying to disable IPv6
I removed the check for IPv6 in the network adaptor. Now I could send mails to freenet.de again. But a few days later we got the same error. Seemed like our hMailServer still used the IPv6 to send emails. Looking into the issue I was about to disable IPv6 for good via the registry.
Setting IP binding in hMailServer
Before restarting the server I checked the hMailServer logs. Turns out that hMailServer tries to send via the IPv4 first, but gets a "457 Greylisted, please come back later" reply. (That is normal behaviour to avoid spams when the triplet (IP, sender, recipient) has not occurred yet.) Only after a few tries hMailServer uses the IPv6 to send, which results in the above mentioned 550 error. Another search brought up a setting in hMailServer where you can explicitly state which IP it should use for sending (Settings/Protocols/SMTP –> Advanced –> "Bind to local IP address"). After configuring that sending emails worked again without problems.