Defaulthost
defaulthost
           Default  host  name.   Default:  me,  if  that is supplied; otherwise the literal name defaulthost, which is probably not what you want.  qmail-inject adds this name to any
           address without a host name.  defaulthost need not be the current host's name.  For example, you may prefer that outgoing mail show just your domain name.
The QMAILDEFAULTHOST environment variable overrides defaulthost.