MX records - 550-Verification failed error

I can't figure this one out.

I have users of one of my domains having trouble getting thier emails to people.  Outgoing emails often get this response:

 ----- The following addresses had permanent fatal errors -----
<sample@sample.com>
    (reason: 550-Verification failed for <test@pactprevention.org>)

   ----- Transcript of session follows -----
... while talking to sample.com.:
>>> RCPT To:<sample@sample.com>
<<< 550-Verification failed for <test@pactprevention.org>
<<< 550-No Such User Here
<<< 550 Sender verify failed
550 5.1.1 <sample@sample>... User unknown
Reporting-MTA: dns; jessica.andrews.com
Received-From-MTA: DNS; [72.243.245.50]
Arrival-Date: Mon, 21 Jul 2008 11:46:03 -0500

The sample@sample.com is a sample of course, but the rest is accurate in case you are able to test this for me.  The actual domain (A records) go to this organzations web master, I handle the email for them, so just the MX records are pointed at my server.

All my other domains are set up the same way, with mail routed to mail.andrews.com, where my server handles incoming mail.  Other domains I have set up the same way as pactprevention work without getting caught with this same error.  I can't find any difference between my domains that work ok and this one that gets the error.

Is someone able to test this or have an idea of what needs changing for the user to be verified correctly?