Bad sequence of commands.

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C#

ASP.NET

I have got this this during mail send:

Bad sequence of commands. The server response was: This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.

SmtpClient sC = new SmtpClient("domain.com");
                sC.Host = ConfigurationManager.AppSettings["servername"];
                sC.Port = 25;
                sC.Credentials = new System.Net.NetworkCredential("username","password");
//smtp.EnableSsl = true;

                sC.Send(mM);