phpNuke problems sending email?

I have a client who has received the following message when his phpnuke tries to send email:

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This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

someusername@yahoo.com
unrouteable mail domain \"yahoo.com\"

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Any idea what could be the problem?

 

 

 

 

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