Si recibes un error del tipo:
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.
<user1@domain.com>:
This message is looping: it already has my Delivered-To line. (#5.4.6)
--- Below this line is a copy of the message.
Return-Path:
Received: (qmail 14418 invoked by uid 110); 9 Jan 2008 13:04:33 -0600
Delivered-To: 54-user2@domain.com
Received: (qmail 14411 invoked by uid 110); 9 Jan 2008 13:04:33 -0600
Delivered-To: 53-user1@domain.com
Received: (qmail 14404 invoked from network); 9 Jan 2008 13:04:33 -0600
Received: from otherdomain.com (HELO otherdomain.com) (11.22.33.44)
by yourmailserver.com with SMTP; 9 Jan 2008 13:04:33 -0600
Significa que tienes un bucle donde user2 redirecciona a user1 a su vez user1 vuelve a redireccionar a user2 y el correo estará jugando al ping pong.
Esto lo puedes solucionar ingresando al panel.elserver.com > Emails > click sobre las casillas que tienen el error y a mano derecha en > Redireccionadores > Eliminas los que aparecen y finalmente click en Guardar cambios.
Saludos