Help Debugging 550 Email Issues

[puddles]

Hi,

My client continues to receive bounce-back messages when sending emails. I've tried to find if they are blacklisted, but have not found one list that they are on. Here are the headers that are being returned (please note that I have replaced the actual names with "user"). Can someone help me debug this and find out why they continue to be bounced back?

Thanks...

-----Original Message-----
From: Mail Delivery System [mailto:[e-mail]]
Sent: Tuesday, October 09, 2007 1:05 PM
To: [e-mail]
Subject: Undelivered Mail Returned to Sender

This is the mail system at host cardero.example1.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<[e-mail]>: host
mx1c9.example3.com[69.156.240.xx]
said: 550 5.7.1 H:MXB<207.7.108.203>Connection refused due to abuse (in
reply to MAIL FROM command)

Reporting-MTA: dns; cardero.example1.com
X-Postfix-Queue-ID: B8A80BB9C3
X-Postfix-Sender: rfc822; [e-mail]
Arrival-Date: Tue, 9 Oct 2007 17:03:59 +0000 (GMT)

Final-Recipient: rfc822; [e-mail]
Original-Recipient: rfc822;[e-mail]
Action: failed
Status: 5.7.1
Remote-MTA: dns; mx1c9.example3.com
Diagnostic-Code: smtp; 550 5.7.1 H:MXB<207.7.108.xx>Connection refused due to
abuse

Received: from 71AEB1941296405 (host-30.example5.com [216.191.164.30])
by cardero.example1.com (Postfix) with ESMTP id B8A80BB9C3
for <[e-mail]>; Tue, 9 Oct 2007 17:03:59 +0000 (GMT)
From: "User" <[e-mail]>
To: <[e-mail]>
Subject:
Date: Tue, 9 Oct 2007 13:04:02 -0400
Message-ID: <008001c80a96$65b08ae0$6901a8c0@71AEB1941296405>
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_0081_01C80A74.DE9EEAE0"
X-Mailer: Microsoft Office Outlook 11
Thread-Index: AcgKlmQM28yfbj3zSx6WD4/4jBaUSg==
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3138

[edited by: jatar_k at 6:18 pm (utc) on Oct. 12, 2007]
[edit reason] no specifics thanks [/edit]

[Romeo]

The administrator of `cardero.example1.com` has somehow decided to not accept mail from your client.

There seems -- besides your lookup of your client in popular DNSBLs -- no further technical debugging possible at your end.

The cardero admin is free to do whatever he may find appropriate to accept or refuse incoming mail traffic.
Perhaps the admin is using his own private blacklist or has implemented other weird filters and special own filter rules.

The only way to solve this would be to write to cardero and ask for the exact reason, or for an explicit whitelisting.

Kind regards,
R.