outbound mail

I have three exchange 2003 servers, server x, server y, server z. Server x is configured to send all external mail from server y and server z. Server y is able to send mail externally through server x but server z isn’t.
When I do a message tracking I get ?the message is routed and queued for remote delivery? and an NDR is returned with error 4.4.7. Server z was able to send mail externally a few days ago. What could be the problem?