from the mail received:
Received: (qmail invoked by alias); 05 Oct 2008 06:30:24 -0000
Received: from cossacks.x10hosting (EHLO cossacks.x10hosting.com) [74.86.116.190]
by mx0.gmx.net (mx043) with SMTP; 05 Oct 2008 08:30:24 +0200
Received: from rk by cossacks.x10hosting.com with local (Exim 4.69)
(envelope-from <rk@cossacks.x10hosting.com>)
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for *-----------* ; Sun, 05 Oct 2008 02:29:59 -0400
To: *---------------*
Subject: Confirm Registration for a Rohobot Kids Account
From:
noreply@rohobotkids.com
Message-Id: <E1KmN8I-0000xp-OS@cossacks.x10hosting.com>
Date: Sun, 05 Oct 2008 02:29:59 -0400
While your visible sender address is
noreply@rohobotkids.com, the envelope sender is
rk@cossacks.x10hosting.com. Now, that server does neither accept mail, nor publish an MX record (send mail destined for me to <server address>), so this means one cannot send mail to that alleged sender. Some recipient servers simply refuse to accept mail in that case - somewhat rightful: once a server accepts mail, it takes responsibility for delivery to the destination or getting a non-delivery report back. Now, the route for the NDR is barred.
Some php installations accept a 5th parameter to mail that could be used to set the envelope sender to an existing account that is able to accept mail.
Others might allow configuration through htaccess files.
Your hosting company should know that.