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;
        };
}

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;
    }
}

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。

有一篇MSDN博客文章是关于调查这类问题的:

故障排除ASP。NET -根据验证过程，远程证书无效: http://blogs.msdn.com/b/jpsanders/archive/2009/09/16/troubleshooting-asp-net-the-remote-certificate-is-invalid-according-to-the-validation-procedure.aspx

这有点晚了，但如果您正在寻找像Yury这样的解决方案，下面的代码将帮助您确定问题是否与自签名证书有关，如果是，则忽略自签名错误。如果需要，显然可以检查其他SSL错误。

我们使用的代码(由Microsoft - http://msdn.microsoft.com/en-us/library/office/dd633677(v=exchg.80).aspx提供)如下:

  private static bool CertificateValidationCallBack(
         object sender,
         System.Security.Cryptography.X509Certificates.X509Certificate certificate,
         System.Security.Cryptography.X509Certificates.X509Chain chain,
         System.Net.Security.SslPolicyErrors sslPolicyErrors)
    {
  // If the certificate is a valid, signed certificate, return true.
  if (sslPolicyErrors == System.Net.Security.SslPolicyErrors.None)
  {
    return true;
  }

  // If there are errors in the certificate chain, look at each error to determine the cause.
  if ((sslPolicyErrors & System.Net.Security.SslPolicyErrors.RemoteCertificateChainErrors) != 0)
  {
    if (chain != null && chain.ChainStatus != null)
    {
      foreach (System.Security.Cryptography.X509Certificates.X509ChainStatus status in chain.ChainStatus)
      {
        if ((certificate.Subject == certificate.Issuer) &&
           (status.Status == System.Security.Cryptography.X509Certificates.X509ChainStatusFlags.UntrustedRoot))
        {
          // Self-signed certificates with an untrusted root are valid. 
          continue;
        }
        else
        {
          if (status.Status != System.Security.Cryptography.X509Certificates.X509ChainStatusFlags.NoError)
          {
            // If there are any other errors in the certificate chain, the certificate is invalid,
         // so the method returns false.
            return false;
          }
        }
      }
    }

    // When processing reaches this line, the only errors in the certificate chain are 
// untrusted root errors for self-signed certificates. These certificates are valid
// for default Exchange server installations, so return true.
    return true;
  }
  else
  {
 // In all other cases, return false.
    return false;
  }
}