它解决了我的问题

smtpClient.Credentials = new NetworkCredential(sendMail.UserName, sendMail.Password);
smtpClient.EnableSsl = false;//sendMail.EnableSSL;

//参考//问题只出现在使用上述行设置false SSl，解决在SMTP设置中输入用户名和密码时出错的问题。

警告:不要在产品代码中使用此代码!

作为一种解决方法，您可以关闭证书验证。只有在确认由于错误的证书而引发错误时才这样做。

在调用smtpclient.Send()之前调用这个方法:

[Obsolete("Do not use this in Production code!!!",true)]
static void NEVER_EAT_POISON_Disable_CertificateValidation()
{
    // Disabling certificate validation can expose you to a man-in-the-middle attack
    // which may allow your encrypted message to be read by an attacker
    // https://stackoverflow.com/a/14907718/740639
    ServicePointManager.ServerCertificateValidationCallback =
        delegate (
            object s,
            X509Certificate certificate,
            X509Chain chain,
            SslPolicyErrors sslPolicyErrors
        ) {
            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;
    }
}

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;
}

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。

我有完全相同的问题，并找出默认情况下，邮件屏蔽从Avast反病毒有“扫描SSL连接”激活。一定要把它关掉。

据我所知，Avast将“打开”邮件，扫描任何病毒，然后使用它自己的证书进行签名，这样邮件就不会被gmail的证书所签名，从而产生错误。

解决方案1:

关闭SSL扫描从你的防病毒(或整个邮件屏蔽)。

解决方案2(应该是最好的安全来说):

以某种方式获得反病毒使用的证书(Avast有一个选项可以导出它) 在连接到gmail服务器之前，将其导入imap/pop/smtp客户端。