Defaulthost

defaulthost

man qmail-inject excerpt:

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.