Mailbox name not allowed. The server response was: sorry, authentication failed or timed out. Please do get messages first to authenticate yourself.(#4.4.3)

Mailbox name not allowed. The server response was: sorry, Authentication failed or timed out. Please do get messages first to authenticate yourself.(#4.4.3)

try
          {
              System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();

              MailMessage mail = new MailMessage();

              mail.To.Add(To);
              mail.From = new MailAddress("emailid");
              mail.Subject = subject;
              mail.Body = body;
              mail.IsBodyHtml = true;
              SmtpClient smtp = new SmtpClient("smtp.rediffmailpro.com", 25);
              smtp.EnableSsl = true;

              smtp.UseDefaultCredentials = false;
              smtp.Credentials = new System.Net.NetworkCredential("username", "password");
              smtp.Send(mail);

              return (1);


          }
          catch (Exception ex)
          {
              TraceService(ex.Message);
              return (0);
          }