查找异常的确切原因 - System.Net.Sockets.SocketException

Question

我尝试使用下面给出的代码向 SMTP 邮件服务器发送邮件。但是，我收到一个错误 -

Error: System.Reflection.TargetInvocationException: Exception has 
been thrown by the target of an invocation. ---> System.Net.Mail.SmtpException:
Failure sending mail. ---> System.Net.WebException: Unable to connect to
the remote server ---> System.Net.Sockets.SocketException: A connection 
attempt failed because the connected party did not properly respond after 
a period of time, or established connection failed because connected host 
has failed to respond 123.456.789:587
at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, 
SocketAddress socketAddress)
at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, 
Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState 
state, IAsyncResult asyncResult, Int32 timeout, Exception& exception)

此异常告诉我可能的原因列表 - 连接 尝试失败，因为连接方在之后没有正确响应 一段时间，或建立连接失败，因为连接的主机 未能响应 123.456.789:587

但它并没有告诉我确切的原因是什么 - 端口号、错误的用户名、密码或其他东西。如何找出确切的原因。

这是我使用的代码 - Sending email with attachments from C#, attachments arrive as Part 1.2 in Thunderbird

using System.Net;
using System.Net.Mail;

public void email_send()
{
    MailMessage mail = new MailMessage();
    SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
    mail.From = new MailAddress("your mail@gmail.com");
    mail.To.Add("to_mail@gmail.com");
    mail.Subject = "Test Mail - 1";
    mail.Body = "mail with attachment";

    System.Net.Mail.Attachment attachment;
    attachment = new System.Net.Mail.Attachment("c:/textfile.txt");
    mail.Attachments.Add(attachment);

    SmtpServer.Port = 587;
    SmtpServer.Credentials = new System.Net.NetworkCredential("your 
    mail@gmail.com", "your password");
    SmtpServer.EnableSsl = true;

    SmtpServer.Send(mail);

}

Answer 1

那个IP地址显然不正确123.456.789:587。
在命令提示符下尝试ping smtp.gmail.com 以查看它是否正在解析到该地址。如果是这样，你在某个地方已经覆盖了它，可能在你的 hosts 文件中（C:\Windows\System32\drivers\etc\hosts）。

在运行你的 sn-p 后，我立即从 Google 收到了“阻止登录尝试”，因为这种身份验证方法显然不符合他们当前的标准。