Relaying Denied

When a outsider send mail to me, she receive an error mail that says the mail cannot be delivered. Below is the error :

The following addresses had permanent fatal errors (reason:550 5.7.1 relaying denied)

Transcript of session follows:

While talking to smtp.cyberway.com.sg
DATA
550 5.7.1 Relaying denied
550 5.1.1 User unknown
503 5.0.0 Need RCPT (recipient)

I’m using Exchange Server 5.5

Any idea what is wrong?

thanks and regards,