[cobalt-users] Mail Problem ERRR!

[Cmopollux@xxxxxxx]

Ok, i feel liek an idiot!  A am a newbie at this so stay with me :)

Ok everythime I try to send an email with outlook express I get the following 
error-

The message could not be sent because one of the recipients was rejected by 
the server. The rejected e-mail address was 'webmaster@xxxxxxxx'. 
Subject 'dfdf', Account: 'Certihost', Server: 'www.2039.com', Protocol: 
SMTP, Server Response: '550 <webmaster@xxxxxxxx>... Relaying denied', 
Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

I don't know what this means and would really appreciate ANY help at all.
Thanks so much :)