Microsoft, Exchange Server 2007, NDR Status 5.0.0 with Error 5.7.1 authentication required for relay

One of my company's clients is receiving non-deliverable receipts when emailing our users.  We have been receiving emails from them for years, but yesterday this started happening.  This is what they are getting back when they send emails to us.  As far as I know they are the only clients who are having problems.  

>>> Client's Name 10/14/2008 7:38 AM >>>

>>> M+ Extreme Email Engine <> 10/14/2008 7:36 AM
>>> M+ >>>
The original message was received Tue, 14 Oct 2008 08:35:27 -0400 from

  ----- The following address had a permanent error -----
    <>; originally to <>
    The mail system encountered a delivery failure, error code -1.
    This failure may be due to circumstances out of the mail systems control.
    ----- the delivery transcript -----
    authentication required for relay (#5.7.1)

Reporting-MTA: dns;
Arrival-Date: Tue, 14 Oct 2008 08:35:27 -0400

Action: failed
Status: 5.0.0

Can someone tell me if this problem is on my side or my client's side and what the fix for it is?  
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michaelgoldsmithConnect With a Mentor Commented:
Check if that client is using a Smart Host to send mail or if they are routing mail via DNS. If they are using a Smart Host, check with that ISP to see if they have a block on your mail server.

If you are receiving mail from everyone EXCEPT that mail server, than this is probably your issue.
JanetKinnairdAuthor Commented:
I was unable to get ahold of their IT department.  The one person I did talk to sent me a test email at 2:42 and it arrived in my inbox at 3:08.  She said they would have to issue a help desk ticket and that the IT dept would not be able to speak to me directly.  Since I received her email I would assume the problem has either cleared up or is intermittent.
Is that the ISP's IT dept or the company that can not send you emails IT dept.
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JanetKinnairdAuthor Commented:
I apologize.  It is the company who is trying to send email to us.  They won't let me talk to their IT department.
They probably figured it out on their end, but you should call your other contacts at that company and have them try to email you to be sure.
JanetKinnairdAuthor Commented:
Thank you for trying to help on this.
JanetKinnairdAuthor Commented:
I didn't know how to grade this since we never really got down to the cause.  I appreciate your help and comments.
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