SMTP服务器进行匿名身份验证时发送邮件

Question

  //   Send anonymous e-mail using SMTP Server (c# code ) 
 //setting SmtpClient property
     SmtpClient smtpClient =new  SmtpClient();
              smtpClient .Host = "smtp.gmail.com";
                    smtpClient .Port = 587;
                    smtpClient .EnableSsl = true;
           smtpClient .DeliveryMethod =SmtpDeliveryMethod.Network;
                smtpClient .UseDefaultCredentials = false;
                smtpClient .Timeout =  30000;

      //setting MailMessage property
              MailMessage mailMessage = new MailMessage();
                 mailMessage.Subject = "test mail";
           mailMessage.From = (new MailAddress("abc@gmail.com"));
                    mailMessage.To.Add("xyz@gmail.com");
                    mailMessage.Priority = MailPriority.High;

                        mailMessage.Body = "Hello";

                      smtpClient.Send(mailMessage);    

基本上，我正在尝试在“身份验证”为“匿名”时发送邮件。如果我通过用户名和密码，则电子邮件发送成功；但是，如果我删除了用户名和密码凭据（即，身份验证为匿名），则无法发送邮件。