message bounced in exchange / #500 5.3.3 Unrecognized command ##

an external user rcvd an NDR and i cant put my finger on why.
copy of NDR below
any ideas? rejected your message to the following e-mail addresses: ( gave this error:
Unrecognized command
The recipient's e-mail system can't process this message. Please try resending the message later.
Diagnostic information for administrators:
Generating server: SERVER01.sender.local #500 5.3.3 Unrecognized command ##
Original message headers:
Received: from SERVER01.sender.local ([fe80::88d9:a57e:832a:52d1]) by
 SERVER01.sender.local ([fe80::88d9:a57e:832a:52d1%11]) with mapi;
 Thu, 2 Sep 2010 16:24:39 -0700
From: Marianne <>
To: ""
Subject: FW: Board Development Committee meeting on Tuesday, September 14,
 2010 at noon
Thread-Topic: Board Development Committee meeting on Tuesday, September 14,
 2010 at noon
Thread-Index: ActFbr6DgtmPHQPJSw6Ojjsad5nu0wEuBu4AAACHBOAAIaFN4AARnBbQ
Date: Thu, 2 Sep 2010 23:24:38 +0000
Message-ID: <C5BA8D71479D194A98702802D130AF6F09E850@SERVER01.sender.local>
Accept-Language: en-US
Content-Language: en-US
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Who is Participating?
Dave StringfellowIT managerCommented:
Quite hard to say without knowing a bit more info, this can be caused by mail scanners, firewalls etc

What do you have between their server and yours?
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are you sure this was a legitimet email?  that error is usually only when the data sent to your server is invalid, like someone sending and email through telnet and they used a backspace or delete key which are invalid characters.
Will SzymkowskiSenior Solution ArchitectCommented:
The NDR 5.3.3 usually indicates that the Remote server has insufficient disk space to hold email.  I would check the SMTP Log.
Dave StringfellowIT managerCommented:
You will usually get "Insufficient resources" if your drives are full.. or at least you do on Exchange 2007
-pHAuthor Commented:
it was a legit email, from somebody who has emailed our domain before.
disk space is fine.
there is an asa5510 and a cisco 3560 core switch between interwebs and our exchange server.
Dave StringfellowConnect With a Mentor IT managerCommented:
You can look here: it has some specific posts about Pix and 5.3.3 errors.
-pHAuthor Commented:
thanks, i applied the no fixup on my asa and will further test
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