Server does not support secure connections in C#
<p>I'm getting the error "Server does not support secure connections" with my code below.</p>

<pre><code>            SmtpClient smtp = new SmtpClient();
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress("*****@gmail.com");
            mail.To.Add(recieverId);
            mail.Subject = "Invoice Copy and Delivery Confirmation for booksap.com Order " + orderno + ". Please Share Feedback.";
            System.Net.Mail.Attachment attachment;
            attachment = new System.Net.Mail.Attachment(Server.MapPath("OrderMail\\Invoice.pdf"));
            mail.Attachments.Add(attachment);
            MailBody = "We are pleased to inform that the following items in your order " + orderno + " have been placed. This completes your order. Thank you for shopping! ";
            StreamReader reader = new StreamReader(Server.MapPath("~/MailHTMLPage.htm"));
            string readFile = reader.ReadToEnd();
            string myString = "";
            myString = readFile;
            myString = myString.Replace("$$Name$$", ContactPersonName);
            myString = myString.Replace("$$MailBody$$", MailBody);
            mail.Body = myString.ToString();
            mail.IsBodyHtml = true;
            mail.Priority = MailPriority.High;
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            smtp.UseDefaultCredentials = true;
            smtp.Port = 587;
            smtp.Credentials = new System.Net.NetworkCredential("*******@gmail.com", "*******");
            smtp.Send(mail);
            mail.Dispose();
            mail = null;
</code></pre>

<p>How can I fix this issue?
If I set</p>

<pre><code> EnabledSsl = false
</code></pre>

<p>it will return the error: <strong>The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated.</strong></p>