IIJ Research Laboratory This document discusses SMTP operational experiences in IPv4/v6 dual stack environments. As IPv6-capable SMTP servers are deployed, it has become apparent that certain configurations of MX records are necessary for stable dual-stack (IPv4 and IPv6) SMTP operation. This document clarifies the existing problems in the transition period between IPv4 SMTP and IPv6 SMTP. It also defines operational requirements for stable IPv4/v6 SMTP operation. This document does not define any new protocol. This memo provides information for the Internet community.