CommonMailServers.cs

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

using MailKit;
using MailKit.Net.Imap;
using MailKit.Net.Pop3;
using MailKit.Net.Smtp;
using MailKit.Security;

namespace CommonMailServerCerts
{
    class Program
    {
        static readonly string[] MailServers = new string[] {
            "imap://imap.gmail.com:993",
            "pop://pop.gmail.com:995",
            "smtp://smtp.gmail.com:587",

            "imap://imap-mail.outlook.com:993",
            "pop://pop-mail.outlook.com:995",
            "smtp://smtp-mail.outlook.com:587",

            "imap://outlook.office365.com:993",
            "pop://outlook.office365.com:995",
            "smtp://smtp.office365.com:587",

            "imap://imap.mail.me.com:993",
            "smtp://smtp.mail.me.com:587",

            "imap://imap.mail.yahoo.com:993",
            "pop://pop.mail.yahoo.com:995",
            "smtp://smtp.mail.yahoo.com:587",

            "imap://imap.gmx.com:993",
            "pop://pop.gmx.com:995",
            "smtp://mail.gmx.com:465",

            "imap://imap.gmx.de:993",
            "pop://pop.gmx.de:995",
            "smtp://mail.gmx.de:465"
        };

        class CertificateInfo
        {
            public string CommonName { get; private set; }
            public string Issuer { get; private set; }
            public string SerialNumber { get; private set; }
            public string Fingerprint { get; private set; }
            public DateTime Expires { get; private set; }

            public CertificateInfo (X509Certificate2 certificate)
            {
                CommonName = certificate.GetNameInfo (X509NameType.SimpleName, false);
                SerialNumber = certificate.SerialNumber;
                Fingerprint = certificate.Thumbprint;
                Issuer = certificate.Issuer;
                Expires = certificate.NotAfter;
            }
        }

        static void Main (string[] args)
        {
            var certificates = new Dictionary<string, CertificateInfo> ();

            foreach (var server in MailServers) {
                var uri = new Uri (server);
                MailService client;
                
                switch (uri.Scheme) {
                case "imap": client = new ImapClient (); break;
                case "pop": client = new Pop3Client (); break;
                case "smtp": client = new SmtpClient (); break;
                default: throw new Exception ("Unsupported protocol");
                }

                using (client) {
                    client.ServerCertificateValidationCallback = delegate (object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) {
                        var info = new CertificateInfo ((X509Certificate2) certificate);

                        if (!certificates.ContainsKey (info.CommonName))
                            certificates.Add (info.CommonName, info);

                        return true;
                    };
                    Console.Write ("Connecting to {0}...", uri);
                    client.Connect (uri.Host, uri.Port, SecureSocketOptions.Auto);
                    client.Disconnect (true);
                    Console.WriteLine ("done");
                }
            }

            Console.WriteLine ("static bool IsKnownMailServerCertificate (X509Certificate2 certificate)");
            Console.WriteLine ("{");
            Console.WriteLine ("\tvar cn = certificate.GetNameInfo (X509NameType.SimpleName, false);");
            Console.WriteLine ("\tvar fingerprint = certificate.Thumbprint;");
            Console.WriteLine ("\tvar serial = certificate.SerialNumber;");
            Console.WriteLine ("\tvar issuer = certificate.Issuer;");
            Console.WriteLine ();
            Console.WriteLine ("\tswitch (cn) {");
            foreach (var kvp in certificates) {
                var info = kvp.Value;

                Console.WriteLine ("\tcase \"{0}\":", info.CommonName);
                Console.WriteLine ("\t\treturn issuer == \"{0}\" && serial == \"{1}\" && fingerprint == \"{2}\"; // Expires {3}", info.Issuer, info.SerialNumber, info.Fingerprint, info.Expires);
            }
            Console.WriteLine ("\tdefault:");
            Console.WriteLine ("\t\treturn false;");
            Console.WriteLine ("\t}");
            Console.WriteLine ("}");
        }
    }
}