Run Postfix in both IPv6 and IPv4

If you want to set your Postfix server to listen on its IPv4 and IPv6 addresses, please add the following line to /etc/postfix/main.cf

inet_protocols = ipv4, ipv6

Restart the server (/etc/init.d/portfix restart) - reload will not work for this setting

Check whether the service is listen on both IPv4 and IPv6
Use netstat -nat command and you should get all services including following lines
tcp        0      0 0.0.0.0:25     0.0.0.0:*                   LISTEN     
tcp        0      0 :::25              :::*                    LISTEN

Note
Hope you have properly configured an IPv6 and IPv4 addresses to the server interface card with proper routing/GW settings