man mailname (Formats) - the visible mail name of the system
NAME
mailname - the visible mail name of the system
DESCRIPTION
The file /etc/mailname is a plain ASCII configuration file, which on a Debian system contains the visible mail name of the system. It is used by many different programs, usually programs that wish to send or relay mail, and need to know the name of the system.
The file contains only one line describing the fully qualified domain name that the program wishing to get the mail name should use (that is, everything after the @).