Brother IntelliFax-1920CN Network User's Manual page 85

In order for the machine to receive data and then send this
information (network fax) to other fax machines the "Relay Broadcast
Function" must be switched "ON". Also you must tell the networked
machine which networks can relay faxes through your machine. This
is known as setting the "Trusted Domain". For instance, if a network
machine has an E-mail address of fax@brother.com and users of
this machine also have E-mail addresses with the same domain
name then the machine must be told to allow these users to send
network faxes through the machine. This is known as the "Relay
Function" and "Trusted Domains". So we set the trusted domain as
brother.com.
"Trusted Domains" Provide security for your machine. In that
only users from a particular domain can send faxes through
your network connected machine.
9 - 6 BROTHER INTERNET FAX PRINTING SOFTWARE