Error in out going mail!

when I triad to send a e-mail from outlook express the next massage appear:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ‘manu327@yahoo.com’. Subject ‘test’, Account: ‘pop.steveduffydesign.com’, Server: ‘smtp.steveduffydesign.com’, Protocol: SMTP, Server Response: ‘550 RCPT TO: Relaying not allowed’, Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

Do I have virus, what is the problem?
I appreciate your time to answered this question.

manu327