Email problem - 501 #5.1.1 bad address

We're getting the following error when mailing a particular email address

User<bob@abc.org> - (501 #5.1.1 bad address bob@abc.org)

The address is valid as i can mail from Hotmail but no one can mail this address from our company.  
We definitely have the right address (no typos) and all other external mail (from our company) is fine.

We're running Exchange 2003 and MailMarshal.

Thanks
kswan_expertAsked:
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r-kConnect With a Mentor Commented:
Check code 5.1.1 in http://support.microsoft.com/default.aspx?scid=kb;en-us;q284204 in case that helps.
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kswan_expertAuthor Commented:

If anyones interested, the problem was resolved by changing the DNS servers for MailMarshal .   The orignal DNS server contained stale records for the domain we were mailing.  Changed to OpenDNS and mail was fine.
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