Hi
I am trying to send a email from .NET it seems to work but nothing happens. After several minutes I get the message
“your email message was unable to be sent because your email server rejected the sender”

   public void SendEmail()
        {
        
        MailMessage mail = new MailMessage();
      mail.From = new MailAddress("fromName@btopenworld.com");           
      mail.To.Add("toName@btopenworld.com");             
      mail.Subject = "hi from stuart ";        	
      mail.Body = "This is the main message";           
SmtpClient smtp = new SmtpClient("mail.btinternet.com");
      smtp.Credentials = new NetworkCredential("myUserName@btopenworld.com", "myPassword");
      smtp.Send(mail);
          
        }

The user name and passwords are correct
What am I doing wrong?
TIA
Stuart