[c#] "The remote certificate is invalid according to the validation procedure." using Gmail SMTP server

I had the same error when i tried to send email using SmtpClient via proxy server (Usergate).

Verifies the certificate contained the address of the server, which is not equal to the address of the proxy server, hence the error. My solution: when an error occurs while checking the certificate, receive the certificate, export it and check.

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);
    }

Full code of my email sender class:

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);
}

}