The Free On-line Dictionary of Computing (30 December 2018):

domainist

    /doh-mayn'ist/ 1. Said of a domain address (as
   opposed to a bang path) because the part to the right of the
   "@" specifies a nested series of "domains"; for example,
   esr@snark.thyrsus.com specifies the machine called snark in
   the subdomain called thyrsus within the top-level domain
   called com.  See also big-endian.

   2. Said of a site, mailer or routing program which knows how
   to handle domainist addresses.

   3. Said of a person (especially a site admin) who prefers
   domain addressing, supports a domainist mailer, or
   proselytises for domainist addressing and disdains bang
   paths.  This term is now (1993) semi-obsolete, as most sites
   have converted.

   [Jargon File]

   (1995-04-21)