Treat IPv6 as a production path, not a checkbox.

Before publishing AAAA or sending over IPv6:

  1. verify routing and firewall rules;
  2. bind SMTP to the intended address;
  3. configure PTR/reverse DNS;
  4. confirm forward DNS;
  5. update SPF where the address is directly authorized;
  6. test inbound/outbound SMTP;
  7. verify TLS;
  8. monitor reputation and failures separately from IPv4.

If IPv6 is broken, remove the advertised path until it is fixed rather than relying on every remote system to fall back cleanly.

Reference material