Some top level domains require you to run at least one secondary nameserver which must be hosted on a different server / IP address than the primary nameserver. You can either set up your own: Guide: How to setup NSD as a secondary nameserver for Mail-in-a-Box
…or you can use a DNS provider that offers secondary name servers as a service, such as Setting up Secondary DNS for Mail-in-a-Box - AnyDomain LLC.
See also here: Mail-in-a-Box Setup Guide