在我们的案例中，问题是由IIS服务器证书引起的。证书的主题设置为DNS名称，用户试图通过IP地址访问网站，因此. net证书验证失败。 当用户开始使用DNS名称时，问题消失。

因此，您必须将提供者URL更改为https://CertificateSubject/xxx/xxx.application

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;
        };
}

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

我的问题是在Windows 2003服务器，当调用AuthenticateAsClient。上面的解决方案(例如绕过ServicePointManager.ServerCertificateValidationCallback)不起作用。

原来这是Windows 2003中的一个bug，并且有一个热修复:

"使用加密API的应用程序无法在Windows Server 2003中验证X.509证书"

https://support.microsoft.com/en-us/kb/938397

安装此修复程序解决了我的问题。

当我试图通过代理服务器(Usergate)使用SmtpClient发送电子邮件时，我有同样的错误。

验证包含服务器地址的证书，该证书不等于代理服务器地址，因此出现错误。 我的解决方案:当检查证书时发生错误，接收证书，导出并检查。

public static bool RemoteServerCertificateValidationCallback(Object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
    {
        if (sslPolicyErrors == SslPolicyErrors.None)
            return true;

        // if got an cert auth error
        if (sslPolicyErrors != SslPolicyErrors.RemoteCertificateNameMismatch) return false;
        const string sertFileName = "smpthost.cer";

        // check if cert file exists
        if (File.Exists(sertFileName))
        {
            var actualCertificate = X509Certificate.CreateFromCertFile(sertFileName);
            return certificate.Equals(actualCertificate);
        }

        // export and check if cert not exists
        using (var file = File.Create(sertFileName))
        {
            var cert = certificate.Export(X509ContentType.Cert);
            file.Write(cert, 0, cert.Length);
        }
        var createdCertificate = X509Certificate.CreateFromCertFile(sertFileName);
        return certificate.Equals(createdCertificate);
    }

我的电子邮件发件人类的完整代码:

public class EmailSender
{
    private readonly SmtpClient _smtpServer;
    private readonly MailAddress _fromAddress;

    public EmailSender()
    {
        ServicePointManager.ServerCertificateValidationCallback = RemoteServerCertificateValidationCallback;
        _smtpServer = new SmtpClient();
    }

    public EmailSender(string smtpHost, int smtpPort, bool enableSsl, string userName, string password, string fromEmail, string fromName) : this()
    {
        _smtpServer.Host = smtpHost;
        _smtpServer.Port = smtpPort;
        _smtpServer.UseDefaultCredentials = false;
        _smtpServer.EnableSsl = enableSsl;
        _smtpServer.Credentials = new NetworkCredential(userName, password);

        _fromAddress = new MailAddress(fromEmail, fromName);
    }

    public bool Send(string address, string mailSubject, string htmlMessageBody,
        string fileName = null)
    {
        return Send(new List<MailAddress> { new MailAddress(address) }, mailSubject, htmlMessageBody, fileName);
    }

    public bool Send(List<MailAddress> addressList, string mailSubject, string htmlMessageBody,
        string fileName = null)
    {
        var mailMessage = new MailMessage();
        try
        {
            if (_fromAddress != null)
                mailMessage.From = _fromAddress;

            foreach (var addr in addressList)
                mailMessage.To.Add(addr);

            mailMessage.SubjectEncoding = Encoding.UTF8;
            mailMessage.Subject = mailSubject;

            mailMessage.Body = htmlMessageBody;
            mailMessage.BodyEncoding = Encoding.UTF8;
            mailMessage.IsBodyHtml = true;

            if ((fileName != null) && (System.IO.File.Exists(fileName)))
            {
                var attach = new Attachment(fileName, MediaTypeNames.Application.Octet);
                attach.ContentDisposition.CreationDate = System.IO.File.GetCreationTime(fileName);
                attach.ContentDisposition.ModificationDate = System.IO.File.GetLastWriteTime(fileName);
                attach.ContentDisposition.ReadDate = System.IO.File.GetLastAccessTime(fileName);
                mailMessage.Attachments.Add(attach);
            }
            _smtpServer.Send(mailMessage);
        }
        catch (Exception e)
        {
            // TODO lor error
            return false;
        }
        return true;
    }

    public static bool RemoteServerCertificateValidationCallback(Object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
        return true;

    // if got an cert auth error
    if (sslPolicyErrors != SslPolicyErrors.RemoteCertificateNameMismatch) return false;
    const string sertFileName = "smpthost.cer";

    // check if cert file exists
    if (File.Exists(sertFileName))
    {
        var actualCertificate = X509Certificate.CreateFromCertFile(sertFileName);
        return certificate.Equals(actualCertificate);
    }

    // export and check if cert not exists
    using (var file = File.Create(sertFileName))
    {
        var cert = certificate.Export(X509ContentType.Cert);
        file.Write(cert, 0, cert.Length);
    }
    var createdCertificate = X509Certificate.CreateFromCertFile(sertFileName);
    return certificate.Equals(createdCertificate);
}

}

使用我的功能诊断证书问题-见屏幕

            System.Net.ServicePointManager.ServerCertificateValidationCallback = Function(s As Object,
                                                                                          cert As System.Security.Cryptography.X509Certificates.X509Certificate,
                                                                                          chain As System.Security.Cryptography.X509Certificates.X509Chain,
                                                                                          err As System.Net.Security.SslPolicyErrors)
                                                                                     Return True
                                                                                 End Function