Are you sure you really need this? What are you trying to achieve? Perhaps there is an easier option.
So, there's domain.com, the NS1 is outside of LAN.
The SME server on LAN also serves this domain locally, because we don't want to configure a.domain.com and b.domain.com on NS1 because of security reasons (to reveal the local network structure) - these servers are in the LAN. "A" records can easily set up in server-manager, but not the "MX"es.
A second SME server has been set up to query the first one (Corporate DNS), and the client machines also (via 2nd server's DHCPd)
So, the 2nd SME server on the network wants to send a mail to log@domain.com, the first SME server mentioned above says the MX server is itself, and it's not good, we can't get the log messages of timed runs, etc..
So the solution was to change the namserver's MX settings for that domain.
But now we can not easily manage the 1st SME server's domain and hostname settings via server-manager thru web.