客户无权提交邮件

Question

这是我的代码：

System.Net.NetworkCredential cred = new System.Net.NetworkCredential("example@gmail.com", "example12345");

System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();
msg.To.Add("friend@gmail.com");
msg.From = new System.Net.Mail.MailAddress("example@gmail.com", "blah");
msg.Subject = "Subject";
msg.Body = "Hello";

System.Net.Mail.SmtpClient client = new System.Net.Mail.SmtpClient("smtp.gmail.com", 25);
client.Credentials = cred;
client.EnableSsl = true;
client.Send(msg); 

错误是：

  

客户端无权向此服务器提交邮件。服务器响应为：5.5.1 STARTTLS可能不会重复。

     在System.Net.Mail.SmtpConnection.GetConnection（String host，Int32 port）的System.Net.Mail.StartTlsCommand.Send（SmtpConnection conn）上的<。System.Net.Mail.StartTlsCommand.CheckResponse（SmtpStatusCode statusCode，String response）在System.Net.Mail.SmtpTransport.GetConnection（字符串主机，Int32端口）的System.Net.Mail.SmtpClient.GetConnection（）处于System.Net.Mail.SmtpClient.Send（MailMessage消息）