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She hasn't had any changes made. They are manually typing the address, and it is correct, but still nothing. 
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Problem was with the sender. Thanks for all of the help.
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Hey, both Mestha and I (although to a greater degree Mestha) pointed to the sender as the issue. &nbsp;You should at least give the points to Mestha.
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Mestha and Zeltron22. I apologize for not giving up the points. n00b mistake...
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We are running Exchange 2007. I have a lone user that has trouble receiving email from another company we do business with. She is able to send email to them, just not receive. She can't receive from anyone sending from that domain. No other users on my Exchange box have any issue sending or receiving from them. Below is the NDR info they receive:<br />
<br />
Your message did not reach some or all of the intended recipients.<br />
&nbsp; &nbsp; &nbsp; Subject: &nbsp;test email<br />
&nbsp; &nbsp; &nbsp; Sent: &nbsp; &nbsp; 3/12/2009 5:01 PM<br />
The following recipient(s) could not be reached:<br />
&nbsp; &nbsp; &nbsp; &lt;My User&gt; on 3/12/2009 5:01 PM<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; The e-mail address could not be found. &nbsp;Perhaps the recipient moved to a different e-mail organization, or there was a mistake in the address. &nbsp;Check the address and try again.<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &lt;their_domain.org #5.1.0&gt;<br />
<br />
I'm trying to determin if this is my issue or theirs. Any help would be greatly appreciated. <br />
The emails they send to me show up in SMTP logs. I don't see a rejection fail or anything else for the instances they try to send to her.<br />

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We are running Exchange 2007. I have a lone user that has trouble receiving email from another company we do business with. She is able to send email to them, just not receive. She can't receive from anyone sending from that domain. No other users on my Exchange box have any issue sending or receiving from them. Below is the NDR info they receive:

Your message did not reach some or all of the intended recipients.
      Subject:  test email
      Sent:     3/12/2009 5:01 PM
The following recipient(s) could not be reached:
      <My User> on 3/12/2009 5:01 PM
            The e-mail address could not be found.  Perhaps the recipient moved to a different e-mail organization, or there was a mistake in the address.  Check the address and try again.
            <their_domain.org #5.1.0>

I'm trying to determin if this is my issue or theirs. Any help would be greatly appreciated. 
The emails they send to me show up in SMTP logs. I don't see a rejection fail or anything else for the instances they try to send to her.


Sender receives NDR 5.1.0

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

Interactions between email servers and clients are governed by email protocols. The three most common email protocols are POP, IMAP and MAPI. Most email software operates under one of these (and many products support more than one).  The correct protocol must be selected, and correctly configured, if you want your email account to work.