Certains message arrivent en mode "code de programmation"

Bonjour,

Depuis quelques temps je reçois de certains correspondants des mails illisibles tels que ci dessous

Return-Path: <xx@gmail.com>
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Date: Tue, 10 Sep 2013 23:23:35 +0200
Message-ID: <CALmM3+XhtQxHH28nxrv80boODYA6D9EH8wpWLiKAGFQrEuCmqg@mail.gmail.com>
Subject: Fwd: xxxxx
From: xx<xx@gmail.com>
To: undisclosed-recipients:;
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Bcc: xx@free.fr

Que faire pour éviter cela car c'est incompréhensible

PS: j'ai remplacé les noms par xx