Database Mail Issue

Hi,

Database mail is configured on sql sever 2005 (9.0.5000). Profile and accounts are created and configured.

When I try to send test message, message never delivered. Don't know where it stuck up. In database mail log there is error:

Message
The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account x (2012-10-31xxx:xx:xx). Exception Message: Cannot send mails to mail server. (Mailbox unavailable. The server response was: 5.7.1 Unable to relay).

Any Help?

Thanks