我有一个方法应该连接到Smtp服务器以查看它是否在线。此方法在测试许多邮件服务器时有效，但不是全部。代码如下，但失败发生在......
client.Connect（strMailServer，intPort）;
...在与服务器交谈的逻辑开始之前。它根本不会连接。我已经确定我正在连接正确的IP和端口（25），并且我使用了像mxtoolbox这样的第三方网站来成功测试相同的服务器IP。这台服务器正在接收来自万维网的常规流量...只有.Net似乎无法连接。我已经查看了防火墙规则，并观看了WireShark以查看服务器上发生了什么，但我从未看到来自我的测试运行的任何传入数据包。防火墙设置为允许所有接口上的端口25的所有连接。
我还使用SmtpClient运行如下所示的类似测试，它也失败了。
var client = new System.Net.Mail.SmtpClient(strMailServer, intPort);
client.Send("test@mydomain.com", "test@mydomain.com", "test message", "This is meant to test an SMTP server to see if it is online, it expects the message to be rejected.");
此处的错误堆栈导致与我的TcpClient尝试相同的欠载错误。 SocketException：{“无法建立连接，因为目标计算机主动拒绝它xxx.xxx.xxx.xxx:25”}
世界上的每个人怎么能够连接到这台服务器...除了我的笔记本电脑......我不认为这是防火墙问题。
帮助！
public static bool TestMailServer(string strMailServer, int intPort, out string strResponse)
{
try
{
try
{
//First I'll try a basic SMTP HELO
using (var client = new TcpClient())
{
client.Connect(strMailServer, intPort);
// As GMail requires SSL we should use SslStream
// If your SMTP server doesn't support SSL you can
// work directly with the underlying stream
using (var stream = client.GetStream())
{
using (var writer = new StreamWriter(stream))
using (var reader = new StreamReader(stream))
{
writer.WriteLine("EHLO " + strMailServer);
writer.Flush();
strResponse = reader.ReadLine();
if (strResponse == null)
throw new Exception("No Valid Connection");
stream.Close();
client.Close();
if (F.StartsWith(strResponse, "220"))
return true;
else
return false;
}
}
}
}
catch (Exception ex)
{
//If the above failed, I'll try with SSL
using (var client = new TcpClient())
{
//var server = "smtp.gmail.com";
//var port = 465;
//client.SendTimeout = 10000;
//client.ReceiveTimeout = 10000;
client.Connect(strMailServer, intPort);
// As GMail requires SSL we should use SslStream
// If your SMTP server doesn't support SSL you can
// work directly with the underlying stream
using (var stream = client.GetStream())
using (var sslStream = new SslStream(stream))
{
sslStream.AuthenticateAsClient(strMailServer);
using (var writer = new StreamWriter(sslStream))
using (var reader = new StreamReader(sslStream))
{
writer.WriteLine("EHLO " + strMailServer);
writer.Flush();
strResponse = reader.ReadLine();
if (strResponse == null)
throw new Exception("No Valid Connection");
stream.Close();
client.Close();
if (F.StartsWith(strResponse, "220"))
return true;
else
return false;
// GMail responds with: 220 mx.google.com ESMTP
}
}
}
}
}
catch (Exception ex)
{
strResponse = ex.Message;
return false;
}
}
答案 0 :(得分：2)
＆＃34;目标机器主动拒绝它＆＃34;意味着没有任何东西在端口25上侦听 - 你发送了一个SYN数据包并且返回了RST而不是SYN / ACK。
如果你甚至没有在wirehark捕获中看到SYN，也许你的笔记本电脑正在正确地解析名称。您是通过名称还是IP连接到服务器？你试过一个简单的
吗？telnet mailhost 25
看看是否连接？
答案 1 :(得分：0)
通常，为防止SMTP服务器发送匿名电子邮件，只有在入站IP地址成功进行反向DNS查找后，他们才会接受连接。我相信您家庭地址的反向DNS查询失败，因此它已断开连接。
答案 2 :(得分：0)
答案很简单我打了个I我意识到......我的ISP阻止了端口25上的所有连接，入站和出站。一旦我搬到另一个地方，一切都运转了。咄!!
对于谁可能想要它，这是我的SMTPTest类，它将探测服务器的SMTP功能。我用它来监控我的服务器，报告中断，并验证用户输入的服务器信息。
public struct SMTPTestResult
{
public string Server;
public bool Found;
public bool OriginalPortSuccess;
public int FinalPort;
public bool UsedSSL;
public string Response;
}
public class SMTPTest
{
public static SMTPTestResult TestMailServer(string MailServer)
{
return TestMailServer(MailServer, 25, true);
}
public static SMTPTestResult TestMailServer(string MailServer, int Port, bool TryOtherPorts)
{
SMTPTestResult result = new SMTPTestResult();
result.Server = MailServer;
if (AttemptMailServer(MailServer, Port, false, out result.Response))
{
//First try the requested port, without SSL.
result.Found = true;
result.UsedSSL = false;
result.OriginalPortSuccess = true;
result.FinalPort = Port;
return result;
}
else if (AttemptMailServer(MailServer, Port, true, out result.Response))
{
//Try the requested port, with SSL.
result.Found = true;
result.UsedSSL = true;
result.OriginalPortSuccess = true;
result.FinalPort = Port;
return result;
}
else if (TryOtherPorts && Port != 465 && AttemptMailServer(MailServer, 465, true, out result.Response))
{
//Try port 465 with SSL
result.Found = true;
result.UsedSSL = true;
result.OriginalPortSuccess = false;
result.FinalPort = 465;
return result;
}
else if (TryOtherPorts && Port != 25 && AttemptMailServer(MailServer, 25, false, out result.Response))
{
//Try port 25, without SSL.
result.Found = true;
result.UsedSSL = false;
result.OriginalPortSuccess = false;
result.FinalPort = 25;
return result;
}
else if (TryOtherPorts && Port != 25 && AttemptMailServer(MailServer, 25, true, out result.Response))
{
//Try port 25, with SSL.
result.Found = true;
result.UsedSSL = true;
result.OriginalPortSuccess = false;
result.FinalPort = 25;
return result;
}
else if (TryOtherPorts && Port != 587 && AttemptMailServer(MailServer, 587, false, out result.Response))
{
//Try port 587, without SSL.
result.Found = true;
result.UsedSSL = false;
result.OriginalPortSuccess = false;
result.FinalPort = 587;
return result;
}
else if (TryOtherPorts && Port != 587 && AttemptMailServer(MailServer, 587, true, out result.Response))
{
//Try port 587, with SSL.
result.Found = true;
result.UsedSSL = true;
result.OriginalPortSuccess = false;
result.FinalPort = 587;
return result;
}
else
{
result.Found = false;
result.OriginalPortSuccess = false;
result.FinalPort = Port;
return result;
}
}
private static bool AttemptMailServer(string strMailServer, int intPort, bool blnSSL, out string strResponse)
{
try
{
if(!blnSSL)
{
//I'll try a basic SMTP HELO
using (var client = new TcpClient())
{
client.Connect(strMailServer, intPort);
using (var stream = client.GetStream())
{
using (var writer = new StreamWriter(stream))
using (var reader = new StreamReader(stream))
{
writer.WriteLine("EHLO " + strMailServer);
writer.Flush();
strResponse = reader.ReadLine();
if (strResponse == null)
throw new Exception("No Valid Connection");
stream.Close();
client.Close();
if (strResponse.StartsWith("220"))
return true;
else
return false;
}
}
}
}
else
{
//I'll try with SSL
using (var client = new TcpClient())
{
client.Connect(strMailServer, intPort);
// As GMail requires SSL we should use SslStream
// If your SMTP server doesn't support SSL you can
// work directly with the underlying stream
using (var stream = client.GetStream())
using (var sslStream = new SslStream(stream))
{
sslStream.AuthenticateAsClient(strMailServer);
using (var writer = new StreamWriter(sslStream))
using (var reader = new StreamReader(sslStream))
{
writer.WriteLine("EHLO " + strMailServer);
writer.Flush();
strResponse = reader.ReadLine();
if (strResponse == null)
throw new Exception("No Valid Connection");
stream.Close();
client.Close();
if (strResponse.StartsWith("220"))
return true;
else
return false;
// GMail responds with: 220 mx.google.com ESMTP
}
}
}
}
}
catch (Exception ex)
{
strResponse = ex.Message;
return false;
}
}
}