Exchange 2007 undeliverable email ( valid email address though)

although this is a valid address i am getting undeliverable. Any ideas ?

ataylor@andytaylorlaw.com
[72.167.238.201] #<[72.167.238.201] #5.0.0 smtp; 5.1.0 - Unknown address error 550-'#5.1.0 Address rejected ataylor@andytaylorlaw.com' (delivery attempts: 0)> #SMTP#

Original message headers:

X-IronPort-AV: E=Sophos;i="4.65,221,1304312400";
   d="scan'208,217";a="9356116"
Received: from unknown (HELO DALMSEX01.lockelord.net) ([172.17.2.14])  by
 DALGNIP01.lockelord.net with ESMTP/TLS/AES128-SHA; 16 May 2011 17:30:06 -0500
Received: from DALMSEX01.lockelord.net ([172.17.2.14]) by dalmsex01
 ([172.17.2.14]) with mapi; Mon, 16 May 2011 17:29:48 -0500
From: "Beatty, J. Robert" <jrbeatty@lockelord.com>
To: 'Dan Miller' <dmiller@msmtx.com>, Amanda Peterson
        <apeterson@andytaylorlaw.com>
CC: 'Andy Taylor ' <ataylor@andytaylorlaw.com>, Paul Tough <ptough@msmtx.com>,
        "'James D. Thompson, III'" <jthompson@velaw.com>, "gsamora@velaw.com"
        <gsamora@velaw.com>, "Loose, Tom F." <tloose@lockelord.com>, "'Michael
 Truncale'" <mjt@obt.com>
Date: Mon, 16 May 2011 17:29:47 -0500
Subject: RE: Oral Argument in Conley v. Comstock Oil
Thread-Topic: Oral Argument in Conley v. Comstock Oil
Thread-Index: AcvJcJ+dRupEiKT1TrWuQDB9rQEA5AApywDgAAA9KCASecBhUAAB/enQAANblQA=
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References: <FA0D4AEA77BB694C8009546D26FE40CE1F50C38673@MSMSERVER1.msm.local>
 <1477820943124641085029583996620@psmtp.com>
 <FA0D4AEA77BB694C8009546D26FE40CE1F50C38799@MSMSERVER1.msm.local>
 <04f701cc1400$99b80030$cd280090$@com>
 <FA0D4AEA77BB694C8009546D26FE40CE258BFD94F9@MSMSERVER1.msm.local>
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