My first important contact with Linux was when I looked for free apps for an Email Server to replace the free version of the AXIGEN (Windows version) and for free OS to replace, where possible, MS Windows Servers.
At that time, I stopped at Zentyal as Email Server (and maybe as replacement for Windows AD).
Since then, I use Zentyal as Email Server for separate domains (even it is not designed for this), to provide email services for my company and for our customers.
In the meantime, I found NethServer (I read about NethServer on the Zentyal Forum) and I started to test it.
When NethServer reached the stable 6.8 version, I decided to test it in production.
For this, I have configured NS 6.8 as another Email Server for separate domains (even it is not designed for this), to provide email services for some of our customers.
The localisation in the network:
Placed in the DMZ Zone (ORANGE) of the UTM.
The access is granted through the Firewall of the UTM (Port Forwarding rules and Outgoing rules).
The hardware:
FUJITSU PRIMERGY RX1330 M1
1 x Intel® Xeon® CPU E3-1220 v3 @ 3.10GHz
8 GB RAM
2 x HDD SATA, 6 Gb/s, 1 TB, 7,200 rpm, 512n, hot-plug, 3.5-inch, business critical
The software:
NethServer release 6.8 (Final), kernel 2.6.32-642.3.1.el6.x86_64
RAID Software configured during NethServer installation
The email groupware:
SOGo groupware, Version 2.3.10
The installed modules (NethServer):
- Backup (Backup of configuration and data) - Backup to an USB Disk, incremental, full backup on Saturday, retention policy: 1 month
- Basic firewall (Configure network adapters and basic firewall)
- DNS and DHCP server (Daemons and tools for DHCP and DNS server)
- Email (Email server and filter)
- Intrusion Prevention System (Monitor and block network traffic for malicious activity)
- MySQL server (Configuration tools for MySQL)
- POP3 connector (Fetch messages from external email accounts with POP3 or IMAP access) - installed as dependencies, not used
- POP3 proxy (Intercept POP3 connections and scan messages for virus and spam)
- SMTP proxy (SMTP proxy)
- SOGo groupware (SOGo server and Thunderbird extensions)
- Statistics (Collect and analyse system statistics)
- Web server (Configuration tools for Apache web server)
The installed modules (Other):
- Fail2ban by Stephane de Labrusse with WHOIS Tool
The Email:
- Configured domains: 107
- Email addresses: 61
- Working email clients for desktop: Outlook 2010, 2013 (POP3 and /or IMAP), Thunderbird (POP3 and /or IMAP)
- Working email clients for mobile phones: Default Email clients for Android, iPhone, Windows Phone (POP3 and /or IMAP)
NOTE:
- Not tested on BlackBerry (only on Zentyal)
- Not tested with Outlook app for Android and iPhone (but tested on Zentyal: didn’t work, as Exchange account type, only worked with POP3 or IMAP account type)
Till now, works everything like a charm! 
EDIT 1:
- Not tested with Outlook app for Android and iPhone (but tested on Zentyal: didn’t work, as Exchange account type (worked but cannot read, open, download or save attachments), only worked with POP3 or IMAP account type).