send an email to multi domains without credentials

Server Error in '/' Application.

Mailbox unavailable. The server response was: 5.7.1 Unable to relay

MailMessage objEmail = new MailMessage();
objEmail.To.Add(txtTo.Text);
objEmail.CC.Add(txtcc.Text);
objEmail.Subject = "Test Email";
objEmail.Body = txtName.Text;
objEmail.Priority = MailPriority.High;
SmtpClient ss = new SmtpClient("mail.Mycompany.com");

ss.Port = 25;
ss.UseDefaultCredentials = true;

try
{
   ss.Send(objEmail);
   Response.Write("Your Email has been sent sucessfully -  Thank You");
}
catch (Exception exc)
{
   Response.Write("Send failure: " + exc.ToString());
}