The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated

System.Net.Mail.MailMessage Msg = new System.Net.Mail.MailMessage();
MailAddress fromMail = new MailAddress("xxx@gmail.com");

Msg.Subject = subject;
Msg.Body = myString.ToString();
Msg.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "xxx-outlook.com";
smtp.Port = 587;
smtp.UseDefaultCredentials = false;
smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
smtp.Credentials = new NetworkCredential(user, pwd);
smtp.EnableSsl = false;
smtp.Timeout = 20000;
smtp.Send(Msg);