The current implementation of IPv6 for the MX does not cover IPv6-only use cases yet. However, that is the path we are on. Your setup does not work since the IPv6 host is not receiving a DNSv6 address. RDNSS is not enabled yet for RAs. When you turn on IPv4 for the host, the host is resolving DNS using DNSv4. This is common for dual-stack setups. Try adding DNSv6 manually for the host while getting the GUA address from the prefix delegation. Please DM me your email if you want to engage further. And thank you for preparing for an IPv6-only future. 🙂 ... View more