If I have understood correctly you request NethServer is already a multi-domain mailserver:
NethServer can handle an unlimited number of mail domains, configurable from the Email > Domains page*
The system enables the creation of an unlimited number of email addresses also known as pseudonyms, from the Email addresses page. Each address is associated with a system user or group owning a mailbox (see User and group mailboxes). It can be enabled on all configured domains or only on specific domains.
Check also http://docs.nethserver.org/en/latest/mail.html#domains
Let me know if it helps