Adding server-Side code to send Email

I have made a form on my web page so that wievers can send e-mail to me directly from my web site. When I test it in the development server, everything i OK, but when I load the application to my hosting (remote) site, I get the following error message: Mailbox name not allowed. The server response was: 5.3.0 <post@dennyeenergien.no>... We do not relay from 216.119.125.63 

Have anybody a clue how to fix this?

Thanks,
Jan Johannessen

PS: The server side code is:  
using System.Net.Mail;
using System.Text;
using System;
using Sys ...