man qmail-inject excerpt:
Plus domain name. Default: me, if that is supplied; otherwise the literal name plusdomain, which is probably not what you want. qmail-inject adds this name to any host name that ends with a plus sign, including defaulthost if defaulthost ends with a plus sign. If a host name does not have dots but ends with a plus sign, qmail-inject uses plusdomain, not defaultdomain.
The QMAILPLUSDOMAIN environment variable overrides plusdomain.