EXCHANGE 2007: Delivery is delayed to these recipients or distribution lists

I'm getting this message when try to send email to 2 particular email addresses, they are able to receive email from my hotmail and I receive back from them.


I'm also receive this message after 1 day or 2:

Delivery has failed to these recipients or distribution lists:
lmurillo@xxxxx.com
Microsoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator.
_____
Sent by Microsoft Exchange Server 2007

Diagnostic information for administrators:
Generating server: exchange-fe.domain.mydomain.com
lmurillo@xxxxx.com
#550 4.4.7 QUEUE.Expired; message expired ##
Original message headers:
Received: from mail.mydomain.com (192.168.1.25) by mail.mydomain.com
(10.0.5.25) with Microsoft SMTP Server (TLS) id 8.1.291.1; Mon, 16 Feb 2009
17:33:41 -0500
Received: from exchange.domain.mydomain.com
([fe80::d954:e837:cec9:bc8e]) by
exchange.domain.mydomain.com
([fe80::d954:e837:cec9:bc8e%10]) with mapi; Mon, 16 Feb 2009 17:33:41 -0500
From: "Wilson S. Henao" <whenao@mydomain.com>
Date: Mon, 16 Feb 2009 17:33:39 -0500
Subject: FW: due diligence letter
Thread-Topic: due diligence letter
Thread-Index: AcmIvJ92x8TjaJ5zRxuwQ91OT1iodAHx6Cuw
Message-ID: <6AF757B3B3DFEC4D912D419ED623D10E0F329F4156@exchange.domain.mydomain.com>
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="_004_6AF757B3B3DFEC4D912D419ED623D10E0F329F4156exchangeinter_"
MIME-Version: 1.0
To: Undisclosed recipients:;
Return-Path: whenao@mydomain.com
Received-SPF: None (exchange-fe.domain.mydomain.com:
whenao@mydomain.com does not designate permitted sender hosts)