All emails to a specific att.net address bounce

[IMT]

One of our small company's employees uses an att.net email address instead of our company.com email address. As she is the lead person in charge of sales, her email working is critical. For the last couple of weeks, however, it's been bouncing. This is the final message we get... any ideas?

Diagnostic information for administrators:

Generating server: OURCOMPANY-SBS.imt.local

her.email@att.net
#550 4.4.7 QUEUE.Expired; message expired ##

Original message headers:

Received: from OURCOMPANY-SBS.imt.local ([fe80::d4d4:1ced:c6dc:a8c]) by
OURCOMPANY-SBS.imt.local ([fe80::d4d4:1ced:c6dc:a8c%18]) with mapi; Thu, 6 Dec
2012 16:26:01 -0500
From: MY NAME<myemail@ourcompany.com>
To: HER NAME "her.email@att.net'"
<her.email@att.net>
Disposition-Notification-To:MYNAME<myemail@ourcompany.com>
Date: Thu, 6 Dec 2012 16:26:00 -0500
Subject: purchase requisition forms...
Thread-Topic: purchase requisition forms...
Thread-Index: Ac3T93k/NXZugfbHTMePBJhIvLpogwAAEhkA
Message-ID: 18BE5FB5EBCBB649973B0EABC63371DF055974427331@OURCOMPANY-SBS.imt.local
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
acceptlanguage: en-US
Content-Type: multipart/mixed;
boundary="_011_18BE5FB5EBCBB649973B0EABC63371DF055974427331OURCOMPANYSBSim_"
MIME-Version: 1.0

 

[EQ Admin]

Hello,

Can other employees send to the same address OK?

Can anyone send to any other ATT email address?

I notice that your mail server is recording IPv6 addresses in the headers, does it really have IPv6 connectivity to the internet?

The example below is From: your-company and To: an ATT address.

What is the IP address of the mail server? Could it be on a blacklist?

That's one reason why ATT might be blocking it but not other companies.

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