port25.com. 3600 IN MX 100 mail.port25.com.
mail.port25.com. 3600 IN AAAA 2002:453f:951e::1
This leads me to issue my last serious warning to all network administrators: 6to4 addresses should not be used to set up web and email servers, whether in test mode or production mode. They cause a lot of troubles. Please use 6in4 tunneling.