Delivery is delayed to these recipients or groups

I'm receiving the below message when sending external emails to SOME people, but not others.

A few things we've noticed:

1. Anyone from our company can send emails to this person and it gets delayed (not restricted to one person in our company)
2. It is affecting multiple domains. The example below is for americaii, but so far we've had at least 15 other people get reported with the same issue this morning.
3. We can still send to other domains (gmail, other company domains, etc.) without issue or delay
4. We are on exchange 2010 and using outlook 2010.
5. We haven't changed anything on our server (no updates, new upgrades or new software)
6. The receipients (e.g., americaii) can still receive email from other senders without issue.
   
   -------------------
   Delivery is delayed to these recipients or groups:
   
   XXXX XXXX (XXXXX@americaii.com) (XXXXX@americaii.com)
   
   YYYY@americaii.com (YYYY@americaii.com)
   
   Subject: ###########
   
   This message hasn't been delivered yet. Delivery will continue to be attempted.
   
   The server will keep trying to deliver this message for the next 22 hours and 52 minutes. You'll be notified if the message can't be delivered by that time.
   
   -------------------------
   
   AFTER 24 hours, we get a rejection notice like this:
   
   -------------------------
   Delivery has failed to these recipients or groups:
   
   XXXXXX@Grainger.com (XXXXXX@Grainger.com)
   The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.
   
   
   
   
   
   
   
   Diagnostic information for administrators:
   
   Generating server: OURSERVER.PTIAssembly.com
   
   XXXXXXX@Grainger.com
   #550 4.4.7 QUEUE.Expired; message expired ##
   
   Original message headers:
   
   Received: from OURSERVER.PTIAssembly.com ([fe80::ec0f:64af:3fcb:1d4b]) by
   OURSERVER.PTIAssembly.com ([fe80::ec0f:64af:3fcb:1d4b%10]) with mapi id
   14.03.0174.001; Tue, 18 Mar 2014 10:16:45 -0500
   From: OUR EMPLOYEE <XXXXX@ptiassembly.com>
   To: "XXXXXX@Grainger.com" <XXXXXX@Grainger.com>
   Subject: SUBJECT LINE
   Thread-Topic: SUBJECT LINE
   Thread-Index: Ac9CvP+ijrm+4l8XRV6cScz5lXX3hA==
   Date: Tue, 18 Mar 2014 15:16:45 +0000
   Message-ID: <1011DE9DA6CCB742A9FEF1F5B667636A524B7EE9@OURSERVER.PTIAssembly.com>
   Accept-Language: en-US
   Content-Language: en-US
   X-MS-Has-Attach:
   X-MS-TNEF-Correlator:
   x-originating-ip: [192.168.1.231]
   Content-Type: multipart/alternative;
               boundary="_000_1011DE9DA6CCB742A9FEF1F5B667636A524B7EE9PTIExchangePTIA_"
   MIME-Version: 1.0