email issues on POP account

Hello all, I have a remote user that is able to send emails to any of the mailboxes that are in our domain, but when she tries to send to an @yahoo.com or any other outside address, she gets the following error:
‘550 sorry, mail to that recipient is not accepted (#5.7.1)’
Any ideas on what setting could be causing this? We currently use GoDaddy as our email host (soon to change)and no one else has this issue.
thanks,
stephen