Bouncehost
From Qmailtoaster
bouncehost
man qmail-send excerpt:
Bounce host. Default: me, if that is supplied; otherwise the literal name bouncehost, which is probably not what you want. If a message is permanently undeliverable, qmail-send sends a single-bounce notice back to the message's envelope sender. The notice is From: bouncefrom@bouncehost, although its envelope sender is empty.