A few email calendar invites bouncing back either with Exchange 2007 or Ironport C160

We have exchange 2007 and use a Cisco Ironport C160 for spam filtering. I have an unusual situation where a specific user is sending calender invites to an affiliate company, and a handful of them are bounding back. We are on different Windows domains but are connected via a VPN. I can ping the other offices devices, but do not have good name resolution. One of our local users is sending calender invites to another user at the affiliate office. Most of them have gone through but a couple have been rejected. I am trying to find out the reason for the rejection as this is causing concern that not all invites are getting to the other office. Below is the output. I need to advice on how to troubleshoot this. Does anyone see anything obvious from the output?

From: Mail Delivery System [mailto:mailer-daemon@electric.net]
Sent: Friday, July 26, 2013 1:47 PM
To: smith
Subject: Mail delivery failed: returning message to sender

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

  MikeB@xxx.com
    retry timeout exceeded

------ This is a copy of the message, including all the headers. ------
------ The body of the message is 96234 characters long; only the first
------ 8192 or so are included here.

Return-path: <smith@yyy.com>
Received: from smtp-out3.electric.net ([72.35.12.187])
      by tls2.electric.net with esmtp (Exim 4.76)
      (envelope-from <smith@yyy.com>)
      id 1V2lWA-0002V8-B2
      for MikeB@xxx.com; Fri, 26 Jul 2013 10:09:26 -0700
Received: from 1V2lW6-0006HN-Vn by skoda.electric.net with hostroute:1400316 (Exim 4.77)
      (envelope-from <yyy.com>)
      id 1V2lWA-0006OV-TR
      for MikeB@xxx.com; Fri, 26 Jul 2013 10:09:26 -0700
Received: by emcmailer; Fri, 26 Jul 2013 10:09:26 -0700
Received: from [64.206.187.230] (helo=popeye.qqq.com)
      by skoda.electric.net with esmtp (Exim 4.77)
      (envelope-from <smith@yyy.com>)
      id 1V2lW6-0006HN-Vn
      for MikeB@xxx.com; Fri, 26 Jul 2013 10:09:24 -0700
Received: from mailmiaca.qqq.com ([192.168.1.184])
  by popeye.qqq.com with ESMTP; 26 Jul 2013 13:09:21 -0400
Received: from MAILFTLCA.qqq.com (192.168.2.184) by MAILMIACA.qqq.com
 (192.168.1.184) with Microsoft SMTP Server (TLS) id 8.3.298.1; Fri, 26 Jul
 2013 13:09:21 -0400
Received: from FTLMAILDB.qqq.com ([::1]) by MAILFTLCA.qqq.com
 ([192.168.2.184]) with mapi; Fri, 26 Jul 2013 13:09:21 -0400
From: smith <smith@yyy.com>
To: "'K, Patricia A'" <patricia_a_k@zzz.com>
CC: Mike B <MikeB@xxx.com>
Date: Fri, 26 Jul 2013 13:09:20 -0400
Subject: RE: Please ADD - Mike B - United Employer Services // EV3463955
Thread-Topic: Please ADD - Mike B- United Employer Services //
 EV3463955
Thread-Index: Ac6JQR6ayaJwnFCeTTOvnXd1XK+PyAABrz/QAAClpmAAACBYoAAAFY7QAADRlvAAA9/jAAAB4k9gAAEsvsAAJSriMAAGd9NAAAJ9OnA=
Message-ID: <118CB515C285D046AFB46E62847699BC010463ACD5AE@FTLMAILDB.qqq.com>
References: <52AAD84EA01DE94FAA29E6C416F0A4BB046CD3@APSWP0475.ms.ds.uhc.com>
 <84A3EB7689BC634EA1B83AC574AA722103B106@APSWP0428.ms.ds.uhc.com>
 <118CB515C285D046AFB46E62847699BC010463ACD586@FTLMAILDB.qqq.com>
 <84A3EB7689BC634EA1B83AC574AA722103B17A@APSWP0428.ms.ds.uhc.com>
 <118CB515C285D046AFB46E62847699BC010463ACD587@FTLMAILDB.qqq.com>
 <84A3EB7689BC634EA1B83AC574AA722103B20F@APSWP0428.ms.ds.uhc.com>
 <118CB515C285D046AFB46E62847699BC010463ACD58A@FTLMAILDB.qqq.com>
 <84A3EB7689BC634EA1B83AC574AA722103B410@APSWP0428.ms.ds.uhc.com>
  <52AAD84EA01DE94FAA29E6C416F0A4BB04710E@APSWP0475.ms.ds.uhc.com>
 <84A3EB7689BC634EA1B83AC574AA722103BC30@APSWP0428.ms.ds.uhc.com>
In-Reply-To: <84A3EB7689BC634EA1B83AC574AA722103BC30@APSWP0428.ms.ds.uhc.com>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
acceptlanguage: en-US
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MIME-Version: 1.0
X-Outbound-IP: 64.206.187.230
X-Env-From: smith@yyy-fl.com
X-PolicySMART: 1657794, 1697616
X-Virus-Status: Scanned by VirusSMART (c)
X-Virus-Status: Scanned by VirusSMART (s)

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