It’s really annoying to see cross marks on status checks on Mail-in-a-Box, it’s simply because your box doesn’t have an IPV6 or it’s been disabled. Having said that, we just simply re-enable it.
sudo sysctl -w net.ipv6.conf.all.disable_ipv6=0
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=0
sudo service nsd start