我的问题不是通过IP地址而不是URL引用服务器。我从CA处购买了一个签名证书，用于在专用网络中使用。在引用服务器时，证书上指定的URL很重要。当我通过证书中的URL引用服务器时，一切都开始工作了。

接受的答案中的代码帮助我调试问题。然后我意识到证书参数的SN字段与我认为的SMTP服务器不相同。通过将SmtpClient实例的Host属性设置为证书的SN值，我能够解决这个问题。

对于那些在使用自签名证书连接到本地站点时遇到同样错误的人，下面的博文帮助我解决了这个问题。

http://brainof-dave.blogspot.com.au/2008/08/remote-certificate-is-invalid-according.html

添加这条线对我很有效。这实际上信任这里提到的所有证书。但是，这主要用于故障排除。如果这适用于您，则意味着远程服务器的证书没有作为受信任证书添加到您的计算机中。

System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(RemoteServerCertificateValidationCallback);

完整代码为

private void sendAMail(String toAddress, String messageBody)
        {
            String msg = "Sending mail to : " + toAddress;

            MailMessage mail = new MailMessage();
            mail.To.Add(toAddress);
            mail.From = new MailAddress("from@mydomain.com");
            mail.Subject = "Subject: Test Mail";
            mail.Body = messageBody;
            mail.IsBodyHtml = true;            

            //Added this line here
            System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(RemoteServerCertificateValidationCallback);
            SmtpClient smtp = new SmtpClient();

            smtp.Host = "myhostname.com";            
            smtp.Credentials = new System.Net.NetworkCredential("sender@sample.com", "");
            smtp.EnableSsl = true;
            smtp.Port = 587;            
            smtp.Send(mail);            
        }


private bool RemoteServerCertificateValidationCallback(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Security.Cryptography.X509Certificates.X509Chain chain, System.Net.Security.SslPolicyErrors sslPolicyErrors)
{
    //Console.WriteLine(certificate);
    return true;
}

您可以通过在证书无效时询问用户是否想继续来改进代码。你想继续吗?如下:

ServicePointManager.ServerCertificateValidationCallback = 
    new RemoteCertificateValidationCallback(ValidateServerCertificate);

并添加一个这样的方法:

public static bool ValidateServerCertificate(object sender,X509Certificate certificate,X509Chain chain,SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
        return true;
    else
    {
        if (System.Windows.Forms.MessageBox.Show("The server certificate is not valid.\nAccept?", "Certificate Validation", System.Windows.Forms.MessageBoxButtons.YesNo, System.Windows.Forms.MessageBoxIcon.Question) == System.Windows.Forms.DialogResult.Yes)
            return true;
        else
            return false;
    }
}

我知道我在这个游戏中已经很晚了，但我还没有看到指向系统的答案。TLS流的诊断日志。

在对代码进行任何更改之前，请确保您了解问题所在。AuthenticationException是一种非常通用的异常，它没有透露太多信息。要了解底层的工作原理，请编辑应用程序的app.config文件(或创建一个新文件)，并确保拥有System。在系统中启用网络跟踪源。诊断部分，例如:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.diagnostics>
    <trace autoflush="true" />
    <sharedListeners>
      <add name="file" initializeData="c:\network.log" type="System.Diagnostics.TextWriterTraceListener" />
    </sharedListeners>
    <sources>
      <source name="System.Net" switchValue="Verbose">
        <listeners>
          <add name="file" />
        </listeners>
      </source>
    </sources>
  </system.diagnostics>
</configuration>

重新运行应用程序并检查c:\network.log文件。您应该会看到关于TLS (SSL)连接的详细信息，例如:

System.Net Information: 0 : [12764] InitializeSecurityContext(credential = System.Net.SafeFreeCredential_SECURITY, context = f44368:535f958, targetName = localhost, inFlags = ReplayDetect, SequenceDetect, Confidentiality, AllocateMemory, InitManualCredValidation)
System.Net Information: 0 : [12764] InitializeSecurityContext(In-Buffers count=2, Out-Buffer length=0, returned code=OK).
System.Net Information: 0 : [12764] Remote certificate: [Version]
  V3

[Subject]
  CN=test
  Simple Name: test
  DNS Name: example.com

[Issuer]
  CN=Root CA
  Simple Name: Root CA
  DNS Name: Root CA

...

[Signature Algorithm]
  sha256RSA(1.2.840.113549.1.1.11)

[Public Key]
  Algorithm: RSA
  Length: 2048
  Key Blob: ....
System.Net Information: 0 : [12764] SecureChannel#38496415 - Remote certificate has errors:
System.Net Information: 0 : [12764] SecureChannel#38496415 -    Certificate name mismatch.
System.Net Information: 0 : [12764] SecureChannel#38496415 - Remote certificate was verified as invalid by the user.
System.Net Error: 0 : [12764] Exception in AppDomain#10923418::UnhandledExceptionHandler - The remote certificate is invalid according to the validation procedure..

知道了问题的原因，您应该能够解决它，或者至少缩小您的谷歌搜索范围。