Salut Gabriel, 
Basically you need to setup NS as you will use it only as a local LAN mail server as David also stated before.
For Firewall and filtering mails you can try to use SOPHOS UTM instead of Endian UTM.
Just give this a try before you decide on a complete setup.
I’m also interested in deploing a replacement for Zentyal. Let’s exchange some info 
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Best regards
Bogdan