Help me with these unroutable emails please

I have got a problem with forums and shopping carts sending mails. whenever they send them to yahoo.co.uk or hotmail.com (they are just two examples) i get a bounced message that the domain is unrouteable. Yet when testing to see if i can send to those domains via outlook express i dont get that message and the email is delivered correctly.
Please help as this is driving me round the bend.

Nick

 

 

 

 

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