[Exim] Refused Relay (Host)

Hi, why someone at hotmail.com can't send email to 'rubin@???'? Why
Exim said refused relay? AFAIK, relay mean using some smtp server to sending
email out and in this case, people send to 'rubin@???' and not using
'ectu.com' as smtp server. How must I fix this problem?

$ tail -f /var/log/maillog
Feb 25 10:42:16 dnspri1 exim[1455]: 2002-02-25 10:42:16 refused relay (host)
to <rubin@???> from <hanlim@???> H=f80.law3.hotmail.com
(hotmail.com) [209.185.241.80]

Thanks,
Han Lin