I stumbled upon this peculiar bounce message recently while working on a server:
Hi. This is the qmail-send program at yourmailserver.com. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out. <email@example.com>: This message is looping: it already has my Delivered-To line. (#5.4.6) --- Below this line is a copy of the message.</p> Return-Path: <firstname.lastname@example.org> Received: (qmail 14418 invoked by uid 110); 9 Jan 2008 13:04:33 -0600 Delivered-To: email@example.com Received: (qmail 14411 invoked by uid 110); 9 Jan 2008 13:04:33 -0600 Delivered-To: firstname.lastname@example.org Received: (qmail 14404 invoked from network); 9 Jan 2008 13:04:33 -0600 Received: from otherdomain.com (HELO otherdomain.com) (184.108.40.206) by yourmailserver.com with SMTP; 9 Jan 2008 13:04:33 -0600
Basically, this is qmail’s way of letting you know that your e-mails are stuck in a mail loop. One e-mail user is redirecting to another e-mail user, and that e-mail user is redirecting back to the first one. If q-mail already has a delivered to line which matches one that it already added, it bounces the e-mail and halts delivery.