550 Administrative prohibition / isolsoft

Hi there

I hope you can help us with a problem we have installing "help center" från isolsoft.
When users sign up or whenever this help desk script is sending email we get following
error message:

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. The following address(es) failed:

support5@xxxxxxxx.nu
SMTP error from remote mailer after end of data:
host oderland.nu [213.xxx.xxx.4]: 550 Administrative prohibition

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

Return-path: <nobody@frejus.xxxxxxxxxx.com>
Received: from nobody by frejus.xxxxxxxx.com with local (Exim 4.43)
id 1CehaQ-0002A1-Q7
for support5@xxxxxxxxxx.nu; Wed, 15 Dec 2004 23:25:34 +0100
To: support5@xxxxxxxxx.nu
Subject: Account activation
From: support@xxxxxxxx.org <support@xxxxxxxxxx.org>
X-Sender: support@xxxxxxxx.org
X-Mailer: xxxxxx HelpDesk
Message-Id: <E1CehaQ-0002A1-Q7@frejus.xxxxxx.com>
Date: Wed, 15 Dec 2004 23:25:34 +0100

That helpdesk script does not have smtp authentication included but it shold not be the issue here. It seems to me that is the remote smtp server that is failing, it is a server on the same network, both of those servers are feodora core 2 servers with latest edge version of cpanel, we upgraded everything during the day but stil no progress.

We tried to activate one user with a external email account , that went just fine. So the problem is on our network with cpanel servers.

Any clue, ?

Thanks in advance