This change opens up what is considered local and what is not local:
where msgHome is the directory where the messaging software was installed, and Instance is the name of the messaging instance (install), often the hostname (short host name).
If you look at the file, one section determines which IP addresses are to be considered internal:
INTERNAL_IP $(188.8.131.52/32) $Y 127.0.0.1 $Y * $N
This file prevents people from using this server to relay messages via SMTP without authenticating as valid users.
Mapping and other MTA configuration files are very picky regarding formatting, including line spacing and indentation. Consult the documentation for details.
The three ...