550 Unrouteable address

I 've gto the following problem with email between two parties.Both parties are our clients so i have access to  both exchange and dns servers. When Party 1 tries to send emai lto Party 2 they get the following:

Your message did not reach some or all of the intended recipients.
Subject: Ad examples
Sent: 30/03/2006 17:06
The following recipient(s) could not be reached:
GB@clear-stone.co.uk on 30/03/2006 17:06
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<IM-Files.Intelligent.local #5.5.0 smtp;550 Unrouteable address>

Any ideas?
Who is Participating?
SembeeConnect With a Mentor Commented:
clear-stone.co.uk isn't setup correctly in DNS.


And the other server is announcing itself as a non-valid domain, so that would be a good place to start.

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