Simplenews keeps sending a email on every cron without recipient

How can I make it stop sending a email on every cron (every hours) and prevent it to happen again?

Thank you.

Here's the mail that is send:

-------- Message original --------
Sujet:	Mail failure - no recipient addresses
Date :	Wed, 02 May 2012 21:00:05 +0200
De :	Mail Delivery System <Mailer-Daemon@ns207945.ovh.net>
Pour :	contact@urbanbees.eu

A message that you sent contained no recipient addresses, and therefore no
delivery could be attempted.

------ This is a copy of your message, including all the headers. ------

Date: Wed, 02 May 2012 21:00:05 +0200
Message-Id: <E1SPemT-00054p-3x@ns207945.ovh.net>
To: 
Subject: Votez pour le concours photo
X-PHP-Originating-Script: 6226:mimemail.module
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="46bc748d2315bfa2089bfec9e295b54f"
Content-Transfer-Encoding: 8Bit
X-Mailer: Drupal
Errors-To: contact@urbanbees.eu
Sender: "Urbanbees" <contact@urbanbees.eu>
From: "Urbanbees" <contact@urbanbees.eu>
Precedence: bulk

This is a multi-part message in MIME format.

--46bc748d2315bfa2089bfec9e295b54f
Content-Type: multipart/related;
    type="multipart/alternative";
    boundary="78f614cd571e0e1556329f61094e7cb0"
Content-Transfer-Encoding: 8bit


--78f614cd571e0e1556329f61094e7cb0
Content-Type: multipart/alternative;
    boundary="6361608b1a2fec3ff702ce0201663365"
Content-Transfer-Encoding: 8bit


--6361608b1a2fec3ff702ce0201663365
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit

Bonjour,

.....