Postfix email error - smtp; 550 Return Path not verifiable

I have user on my system that is trying to send an email but it keeps getting returned/rejected by the recipient:

Here is the error message from the MAILER-DAEMON:

Reporting-MTA: dns;
X-Postfix-Queue-ID: 1E15B1DD616
X-Postfix-Sender: rfc822;
Arrival-Date: Tue, 11 Nov 2008 08:53:32 -0500 (ECT)

Final-Recipient: rfc822;
Original-Recipient: rfc822;
Action: failed
Status: 5.0.0
Remote-MTA: dns;
Diagnostic-Code: smtp; 550 Return Path not verifiable

Is this on my end?

User has been able to send messages to this address before, but not since yesterday.
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The error message may actually be perfectly correct. Is the "FROM" field correct (the return address is on the same domain as your server says it belongs to)? Also there may be problems with reverse DNS or the HELO phrase of your server? If the receiving end implemented some sort of SPAM-fighting system just now, you may have a problem that suddenly manifests itself.

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AAIAdminAuthor Commented:
It ended up that they implemented some SPAM filters on the receiving system end.  I had them put in an exception and the emails should go through now.  Thanks.
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