The dns_ip6_lookup feature is disabled by default as of InterWorx Version 5.1.7. This means
mail will not try to deliver via IPv6, unless the following is completed:
From the command line, /var/qmail/control/dns_ip6_lookup and /var/qmail/control/senderip6 must be created.
To create /var/qmail/control/dns_ip_lookup, run the following command:
echo 1 > /var/qmail/control/dns_ip6_lookup y:iwtest.net:10.1.0.1Example /var/qmail/control/senderip6 entry: iwtest.net:2001:db8:0:1:1:1:1:1
Following these steps will allow mail to be sent via IPv6.