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mailname - the visible mail name of the system
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 @).