#5.1.1 smtp;550 5.1.1 User unknown

Your message did not reach some or all of the intended recipients.

      Subject:      PanphonicsUSA shipment invoice
      Sent:          7/15/2009 5:17 PM

The following recipient(s) could not be reached:

      slaureano@d-and-p.com on 7/15/2009 5:18 PM
            The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.
            <DOBBS-EXG.dobbsstanford.com #5.1.1 smtp;550 5.1.1 <slaureano@d-and-p.com>... User unknown>
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Alan HardistyConnect With a Mentor Co-OwnerCommented:
If you receive a mail from them and reply to them, then it is not a case of wrong details, but more a case of poor configuration at  their end.
They may be using a 3rd party mail filtering host and they have forgotten to ask to add this user to the 3rd party server.
Heir domain only shows 1 MX record for their domain, so this could either be a 3rd party or their own server.  If it is their own server, then they may not have configured it properly or if it is the 3rd party server, then they have forgotten to add this address to the list.
orionnetworkingAuthor Commented:
This occurs when sending outbound from our Exchange server.  It will occassionally work to the same address that we receive this error from.  This is no the only external address that we receive the error for.

Most mail traffic works normally inbound and outbound.  We do not have an outbound filter.  This is an Exchange 2003 server.

Kerem ERSOYPresidentCommented:
ok what is the question ??

If you ask what does it mean: It measn taht you're trying to send a mail to someone but there's no person with that name exists at the receiving end.

But if you are sure that you did not send such a message then it means someone probably a virus is faking your name as a sender and sending this person a mail but there's no such a person there. When an e-mail virus infests a sytem it copies itself and gets the addresbook of the infected sytem and selects persons from it randomly as senders and recipients. This means that  if the receiver is not a valid user at that company it will respond that there's no such user there and this is the notification. It also means that the sender of this email company is not avware of SPAM and back-scatter. This is back-scatter and considered as bad as spamming. An e-mal server should respond to the sender during the initial conversation not accept it then return it as this server does.
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orionnetworkingAuthor Commented:
This is not an invalid email address.  We received a message from them and replied to the message and it worked.  The next time we sent a new message to the address, we received this error.  We have seen this in the past as well.  We occassionally have the error we email addresses that we have sent in the past many times.  

The question is why do we occassionally receive this message?

Ok so does it only happen with the one address?

It could just be the outlook email address cache if thats the case.  

Kerem ERSOYPresidentCommented:
As alanhardisty points out it seems that there's a configuration error at their side. There's nothing you can do about it. When your SMTP server wants to deliver e-mail to their site the server responds that this is not a valid address. So this is not your server's fault and you can do nothing about it.

All you can do is to contact the firm and notify them about the problem.
Alan HardistyCo-OwnerCommented:
My comment 24867057 I feel has answered this question, and it seems that KeremE agrees with me in his comment 24867519
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