v4 SMTP server can send out v6 outgoing email

Some friends asked me the myth about my SMTP server running on IPv4 can send out IPv6 emails to dual-stack SMTP servers.   There is no secret. My SMTP Server does not bind to any specific IP address so it just listen and use all available addresses in the Network Interface Card, both v4 and v6.  In sending email to a dual-stack mail server, v6 path is logically selected first.  However, I can not receive incoming emails from v6 path due to the lack of a MX record pointing to a v6 host.  Interesting stuff !!