Sorry for the delay, and something wrong with my English
First, I managed to configure the server as authoritative.
In fact the Zentyal has a different way of treating the subject.
First, you must configure this data in the option: Infrastructure -> DNS.
The first thing there is to register a host machine. Here the translation for my language leads to a wrong conclusion. In English they call the "hostname", which makes more sense. Actually refers to the name of the host machine.
You can have more than one host. In the same place as you have set the alias of your host, such as; mx.mydomain.com, ns1.mydomain.com, ftp.mydomain.com, etc ... Names that you use for your e-mail server, your DNS server, your FTP server, for example.
After this you can go to the "Name Servers" option. There you determine who is the host of your domain. (ns1.mydomain.com if this was your choice of alias to your DNS server)
You do not have a clear way to define the "master " and "slave"of his domain. Actually I supposed to be the order of the list of hostnames registered there. The higher up would be the master and soon to follow would be the slave. This way worked.
For your e-mail server you will need to register a new hostname with the name of your e-mail, and if you want to use aliases, register an alias for it, and yes now you can set the option "Mail Exchangers ". I suposed that your e-mail server is an other machine.
I did not have this difficulty because my email server is the same machine, but I used alias for it and registered the alias on the "Mail Exchangers". It´s work too.
I Hope this will be helpful.