Mail delivery failed: returning message to sender en boucle

Bonjour,

Suite à l'envoi d'un mail avec une pièce jointe (PDF) nous recevons un MailerDeamon (erreur 552, taille trop grande). Cela fait environ deux mois, et c'est TOUJOURS le même.
Il y a eu une "pause" durant plusieurs semaines, mais depuis 15 jours, c'est reparti.
J'ai déjà cherché de nombreuses fois sur internet, mais je ne trouve pas de solution.

J'utilise RoundCube, et je suis hébergé par phpnet.org (mail, site internet etc...). Je les ai déjà appelé, mais ils m'ont dis que le problème n’était de chez eux.

Merci d'avance pour votre aide.

Voici le mail retour :

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This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error.

Reason: This message was too large (SMTP error 552).

The following address(es) failed:
nemo@localhost
   SMTP error: 552 5.3.4 Message size exceeds fixed limit
Reporting-MTA: dns; carl

Final-Recipient: rfc822; nemo@localhost
Last-Attempt-Date: Sun, 20 Dec 2015 12:46:06 +0100 (CET)
Action: failed
Status: 5.3.4
Diagnostic-Code: 552 5.3.4 Message size exceeds fixed limit
Delivered-To: <***@***.org>
Received: from exim-proxy-1.online.net ([10.42.2.124])
    by exim-backend-17.online.net (Dovecot) with LMTP id 0dlEBQ3SG1auNgAAi1FXbQ
    for <***@***.org>; Mon, 12 Oct 2015 17:31:05 +0200
Received: from [10.42.2.120] (helo=vercors.phpnet.org)
    by exim-proxy-1.online.net with esmtp (Exim 4.76)
    (envelope-from <***@***.com>)
    id 1Zlf44-0005za-67
    for ***@***.org; Mon, 12 Oct 2015 17:31:05 +0200
Received: from vercors.phpnet.org ([46.255.160.222])
    by mx-vit.online.net (MXproxy) for ***@***.org ;
    Mon, 12 Oct 2015 17:09:54 +0200 (CEST)
X-ProXaD-SC: state=HAM score=40
Received: from mailfilter2.phpnet.org (mailfilter2.phpnet.org [195.144.11.141])
    by vercors.phpnet.org (Postfix) with SMTP id 4B8576B61E04
    for <***@***.org>; Mon, 12 Oct 2015 17:30:53 +0200 (CEST)
Received: (qmail 2068 invoked by uid 0); 12 Oct 2015 15:30:53 -0000
Received: from unknown (HELO smtp.phpnet.org) (194.110.192.60)
  by mailfilter2.phpnet.org with SMTP; 12 Oct 2015 14:59:51 -0000
Received: from ***-144-11-66.phpnet.fr ([195.144.11.66] helo=webmail.phpnet.org)
    by smtp.phpnet.org with esmtp (Exim 4.72)
    (envelope-from <***@***.com>)
    id 1ZleZp-0005WT-9F; Mon, 12 Oct 2015 16:59:50 +0200
MIME-Version: 1.0
Content-Type: multipart/mixed;
 boundary="=_05096c62be7468f9bd3766c45c7c9186"
Date: Mon, 12 Oct 2015 16:59:32 +0200
From: B*** V*** <***@***.com>
To: undisclosed-recipients: ;
Subject:***
Message-ID: <0bcd45b2ea9ad88ffecdcf8a1cfee6f9@***.com>
X-Sender: ***@***.com
User-Agent: Roundcube Webmail/1.1.1
ScriptPath:
EximSenderAddress: ***@***.com
X-online-proxy-in: FLEX_a0FLeU1VTVFxWVdXc1FTOA==tcN09gdYA0VWGWQdnxwH8FarKWqx
X-online-to: ***@***.org
X-original-for: ***@***.org

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