Defaultdomain
From Qmailtoaster
defaultdomain
man qmail-inject excerpt:
Default domain name. Default: me, if that is supplied; otherwise the literal name defaultdomain, which is probably not what you want. qmail-inject adds this name to any host name without dots, including defaulthost if defaulthost does not have dots. (Exception: see plusdomain.)
The QMAILDEFAULTDOMAIN environment variable overrides defaultdomain.