Requested action not taken: mailbox unavailable. [SMTP Error Code 550]

Mails I send through SMTP gateway (WebShield SMTP gateway) are not being delivered and are coming back to me with the following.
What's going wrong ?


From: Webshield SMTP V4.5 MR1a Mail Service [mailto:Webshield SMTP V4.5 MR1a Mail Service]
Sent: 28 April 2004 19:54
To: nicolas.vincent@cambrex.com
Subject: Returned Mail: Error During Delivery

------ Here is your List of Failed Recipients ------
<nicolasvincent@skynet.be>


Requested action not taken: mailbox unavailable. [SMTP Error Code 550]

-------- Here Is Your Returned Mail --------
Received: FROM verlap04 BY verws017 ; Wed Apr 28 17:54:07 2004 +0200
Date: Wed, 28 Apr 2004 17:58:13 +0200
From: Supervisor <nicolas.vincent@cambrex.com>
To: Nicolas Vincent <nicolasvincent@skynet.be>
Message-ID: <QURSWEdSWCNBW0cyIFc8ODQ0NzgxNDM@verlap04>
Mime-Version: 1.0
Organization: Cambrex Corporation
X-Mailer: GoldMine [6.50.31113]
Content-Type: text/plain
Content-Transfer-Encoding: quoted-printable

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