why am I getting bounce backs from external email server?

I have 2 users getting the following message from the same external email server for 6 recipients:

This is a delivery status notification, automatically generated by MTA domain1 on Thu, 24 Jan 2013 14:16:18 -0500 Regarding recipient(s) : user1@domain1.com Delivery status : Failed. None of the mail servers for the destination domain have so far responded. (domain1.com).
We have been attempting delivery for 5 times.
Maximum delivery tries attempted. Please contact your administrator to contact the destination domain or resend your message. Delivery failed.
Message Information:
      Sender     : user1@domain2.com
      Recipient  : user1@domain1.com
MTA Response :510 The original message headers are included as attachment.
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ID10TzAsked:
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BlueComputeConnect With a Mentor Commented:
None of the mail servers for the destination domain have so far responded


Misconfiguration at their end if this is being reported by the domain1 mail server.  Does the email get accepted if you try and test using telnet: http://support.microsoft.com/kb/153119
?
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