In the distributed main.cf and main.cf.sample, the inet_protocols is set to "ipv4". That means, "work with IPv4 only." There are other options available (including "ipv4, ipv6" but I think it should be updated to "any", so that it works with IPv4 and IPv6 *when* available. Ref: http://www.postfix.org/IPV6_README.html