man qmail-inject excerpt:
Host name for Message-IDs. Default: me, if that is supplied; otherwise the literal name idhost, which is certainly not what you want. idhost need not be the current host's name. For example, you may prefer to use fake host names in Message-IDs. However, idhost must be a fully-qualified name within your domain, and each host in your domain should use a different idhost.
The QMAILIDHOST environment variable overrides idhost.