Question : Why did that email come back to me?

Hey guys

I've got a question. One of my clients has an email address [email protected] Recently they told me that some people cannot send emails to them, that the emails sent to this address bounce back. At the moment there is 1 external address what cannot send to [email protected]. Otherwise he has no problems with sending or receiving emails.
So here is the information that 1 specific email address gets back from server when they try to send to [email protected].
----------------------------------------------------------------------
From: [email protected]-media.co.uk
[mailto:MAILER-DAEMON@www1.contact-media.co.uk]
Sent: 17 September 2009 09:48
To: Phil Sender
Subject: failure notice

Hi. This is the qmail-send program at www1.contact-media.co.uk.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

:
This address no longer accepts mail.

--- Below this line is a copy of the message.

Return-Path: y.co.uk>
Received: (qmail 18939 invoked from network); 17 Sep 2009 09:48:09 +0100
Received: from exchange.senderscomapny.co.uk (HELO
servername.senderscompany.local) (111.22.82.74)
  by 95185-www1.server1.contact-media.co.uk with SMTP; 17 Sep 2009
09:48:08 +0100
Content-class: urn:content-classes:message
Return-Receipt-To: "Phil Sender" y.co.uk>
MIME-Version: 1.0
Content-Type: multipart/alternative;
      boundary="----_=_NextPart_001_01CA3773.866EF162"
Subject: Meeting with Adam Someone from Somewhere
Disposition-Notification-To: "Phil Sender"
y.co.uk>
X-MimeOLE: Produced By Microsoft Exchange V6.5
Date: Thu, 17 Sep 2009 09:47:44 +0100
Message-ID:
<5A907F4C78016B41ADC80B3F41DAE29A25D25F@servername.senderscomapny.local
>
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
Thread-Topic: Meeting with Adam Someone from Somewhere
Thread-Index: Aco3c4RNf++SZXe0RnWb38uynHoUAQ==
From: "Phil Sender" y.uk>
To: "Martin Receiver"

This is a multi-part message in MIME format.

------_=_NextPart_001_01CA3773.866EF162
Content-Type: text/plain;
      charset="us-ascii"
Content-Transfer-Encoding: quoted-printable

Any ideas why this 1 external email address canno send emails to [email protected]?
[email protected] is just an ordinary POP3 account hosted on 1&1.

Regards,

Chaplin35

Answer : Why did that email come back to me?

The message is being generate by their server, not by yours.
It isn't an Exchange message.

Many ISPs will use a cluster of email servers for their inbound email. As Exchange caches DNS lookup results it is perfectly possible for the server to continue to attempt to deliver to a non-valid server. You would need to look at SMTP logs to see what IP address the message delivery is being attempted to, and then compare it to the MX records found through a DNS lookup both on the server and using a public nslookup service to see if the same results come back.

Simon.