IIJ Research Laboratory This document lists operational requirements for IPv6 SMTP and IPv6-capable MX DNS records. As IPv6 SMTP servers are deployed, it has become apparent that certain configurations are necessary in IPv6-capable MX DNS records for stable dual-stack (IPv4 and IPv6) SMTP operation. This document clarifies the problems that exist in the transition period between IPv4 SMTP and IPv6 SMTP. It also defines operational requirements for stable IPv4/v6 SMTP operation.