Sorry for the late response.
I am currently facing an issue that OpenSMTPd disables a route for a dual-stack MX if a connection to the v4 address fails, without retrying v6.The system has a local v4 network, but no routes; A v6 default route is set and the system is reachable via IPv6.
Have you tried forcing IPv6 with a IPv6 src address at the relay action? action "outbound" relay src $v6adr
