Not sure whether this is a true NethServer howto.
More an OS X/macOS howto, since NethServer provides standards based services:
- IMAP using Dovecot;
- SMTP using Postfix;
- CalDAV and CardDAV using SOGo.
These services are supported by OS X/macOS using Mail.app, Calendar.app and Contacts.app (or several third party equivalents).
But NethServer is a proper OS X server replacement!