gnus-local-domain

gnus-local-domain is a variable defined in `gnus.el'.
Its value is

nil

• This variable is obsolete since Emacs 24.1.

Documentation:
Local domain name without a host name.
The DOMAINNAME environment variable is used instead if it is defined.
If the function `system-name' returns the full Internet name, there is
no need to set this variable.

You can customize this variable.