Is there any way to configure OpenSMTPd to relay to another mail server only for outgoing emails to MXs that do not have IPv6 native addresses? My current site has working IPv6, but the IPv4 connectivity is provided by NAT64 which is then natted again by a CG-NAT. I am not able to control the external CG-NAT address and it does not have a PTR record set. IPv6 outgoing mail works just fine. My ISP does not have any more IPv4 addresses to give out.
It would be very helpful if there was a way to relay outgoing mail to IPv4 hosts only, instead of all or nothing. Currently I can only figure out how to configure this to do an all or nothing. -- Thank you, Thomas Groman
