MailServiceServerCertificateValidationCallback Property

public RemoteCertificateValidationCallback ServerCertificateValidationCallback { get; set; }

using System;
using System.Net.Security;
using System.Collections.Generic;
using System.Security.Cryptography.X509Certificates;

using MimeKit;
using MailKit;
using MailKit.Security;
using MailKit.Net.Smtp;

namespace MailKit.Examples
{
    public static class SslCertificateValidationExample
    {
        public static void SendMessage (MimeMessage message)
        {
            using (var client = new SmtpClient ()) {
                // Set our custom SSL certificate validation callback.
                client.ServerCertificateValidationCallback = MySslCertificateValidationCallback;

                // Connect to smtp.gmail.com on the SSL-wrapped port.
                client.Connect ("smtp.gmail.com", 465, SecureSocketOptions.SslOnConnect);

                // Authenticate with our username and password.
                client.Authenticate ("username@gmail.com", "password");

                // Send our message.
                client.Send (message);

                // Disconnect cleanly from the server.
                client.Disconnect (true);
            }
        }

        static bool MySslCertificateValidationCallback (object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {
            // If there are no errors, then everything went smoothly.
            if (sslPolicyErrors == SslPolicyErrors.None)
                return true;

            // Note: MailKit will always pass the host name string as the `sender` argument.
            var host = (string) sender;

            if ((sslPolicyErrors & SslPolicyErrors.RemoteCertificateNotAvailable) != 0) {
                // This means that the remote certificate is unavailable. Notify the user and return false.
                Console.WriteLine ("The SSL certificate was not available for {0}", host);
                return false;
            }

            if ((sslPolicyErrors & SslPolicyErrors.RemoteCertificateNameMismatch) != 0) {
                // This means that the server's SSL certificate did not match the host name that we are trying to connect to.
                var certificate2 = certificate as X509Certificate2;
                var cn = certificate2 != null ? certificate2.GetNameInfo (X509NameType.SimpleName, false) : certificate.Subject;

                Console.WriteLine ("The Common Name for the SSL certificate did not match {0}. Instead, it was {1}.", host, cn);
                return false;
            }

            // The only other errors left are chain errors.
            Console.WriteLine ("The SSL certificate for the server could not be validated for the following reasons:");

            // The first element's certificate will be the server's SSL certificate (and will match the `certificate` argument)
            // while the last element in the chain will typically either be the Root Certificate Authority's certificate -or- it
            // will be a non-authoritative self-signed certificate that the server admin created. 
            foreach (var element in chain.ChainElements) {
                // Each element in the chain will have its own status list. If the status list is empty, it means that the
                // certificate itself did not contain any errors.
                if (element.ChainElementStatus.Length == 0)
                    continue;

                Console.WriteLine ("\u2022 {0}", element.Certificate.Subject);
                foreach (var error in element.ChainElementStatus) {
                    // `error.StatusInformation` contains a human-readable error string while `error.Status` is the corresponding enum value.
                    Console.WriteLine ("\t\u2022 {0}", error.StatusInformation);
                }
            }

            return false;
        }
    }
}