// This is an Exchange 2013 custom transport agent for sender based re-routing, created by Martin Tuescher
// As it is here, it needs a Send Connector configured to route mails to override domains that has equal or
// lower costs than the Default Send Connector for "*".

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Configuration.Install;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using Microsoft.Win32;

using Microsoft.Exchange.Data.Transport;
using Microsoft.Exchange.Data.Transport.Email;
using Microsoft.Exchange.Data.Transport.Smtp;
using Microsoft.Exchange.Data.Transport.Routing;
using Microsoft.Exchange.Data.Common;

namespace Microsoft.Exchange.SBR
{
    /// <summary>
    // Run when installing the assembly, this function sets up the required application event logging registry keys
    /// </summary>
    [RunInstaller(true)]
    public class SbrRoutingAgentEventLogInstaller : Installer
    {
        private EventLogInstaller SbrEventLogInstaller;

        public SbrRoutingAgentEventLogInstaller()
        {
            SbrEventLogInstaller = new EventLogInstaller();			// Create an instance of an EventLogInstaller.
            SbrEventLogInstaller.Source = "Microsoft.Exchange.SBR";	// Set the source name of the event log.
            SbrEventLogInstaller.Log = "Application";				// Set the event log that the source writes entries to.
            Installers.Add(SbrEventLogInstaller);					// Add routingeventLogInstaller to the Installer collection.

            RegistryKey newRegKey = Registry.LocalMachine.CreateSubKey("System\\CurrentControlSet\\Services\\MSExchangeSbrAgent\\Diagnostics");
            newRegKey.SetValue("General", 0);
        }
    }

    public sealed class SbrRoutingAgentFactory : RoutingAgentFactory
    {
        public override RoutingAgent CreateAgent(SmtpServer server)
        {
            RoutingAgent myAgent = new OwnRoutingAgent();
            return myAgent;
        }
    }

    public class OwnRoutingAgent : RoutingAgent
    {
        private int eventLogLevel = -1;     // Don't log if the proper keys aren't setup because this could cause problems later.
        private RegistryKey eventLogRegKey;

        private Dictionary<String, String> _RoutingTable;
        private String OverrideSettingsFileName = Environment.GetEnvironmentVariable("ExchangeInstallPath", EnvironmentVariableTarget.Machine) + @"TransportRoles\agents\Custom\Microsoft.Exchange.SBR.OverrideSettings.config";

        private String[] _InternalDomains;
        private String InternalDomainsFileName = Environment.GetEnvironmentVariable("ExchangeInstallPath", EnvironmentVariableTarget.Machine) + @"TransportRoles\agents\Custom\Microsoft.Exchange.SBR.InternalDomains.config";

        private String[] _IgnoreAuthAsValues;
        private String IgnoreAuthAsFileName = Environment.GetEnvironmentVariable("ExchangeInstallPath", EnvironmentVariableTarget.Machine) + @"TransportRoles\agents\Custom\Microsoft.Exchange.SBR.IgnoreAuthAs.config";

        public OwnRoutingAgent()
        {
            // Read EventLogLevel from Registry
            this.eventLogRegKey = Registry.LocalMachine.OpenSubKey("SYSTEM\\CurrentControlSet\\services\\MSExchangeSbrAgent\\Diagnostics");
            if (this.eventLogRegKey != null)
            {
                if (eventLogRegKey.GetValue("General") != null)
                {
                    this.eventLogLevel = ((int)eventLogRegKey.GetValue("General"));

                    if (eventLogLevel >= 6) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Event logging set to level " + this.eventLogLevel.ToString() + ".");
                }
            }

            //subscribe to different events
            base.OnResolvedMessage += new ResolvedMessageEventHandler(OwnRoutingAgent_OnResolvedMessage);

            _IgnoreAuthAsValues = File
                                  .ReadAllLines(IgnoreAuthAsFileName)
                                  .Where(line => !String.IsNullOrEmpty(line))
                                  .Where(line => !line.StartsWith("#"))
                                  .Select(line => line.ToLower())
                                  .ToArray();   // assumes line has 1 element, no error check
            
            _InternalDomains = File
                               .ReadAllLines(InternalDomainsFileName)
                               .Where(line => !String.IsNullOrEmpty(line))
                               .Where(line => !line.StartsWith("#"))
                               .Select(line => line.ToLower())
                               .ToArray();

            _RoutingTable = File
                            .ReadAllLines(OverrideSettingsFileName)
                            .Where(line => !String.IsNullOrEmpty(line))
                            .Where(line => !line.StartsWith("#"))
                            .Select(line => line.ToLower())
                            .Select(line => line.Split(';'))
                            .ToDictionary(items => items[0], items => items[1]); // assumes line has 2 elements, no error check
        }

        private void OwnRoutingAgent_OnResolvedMessage(ResolvedMessageEventSource source, QueuedMessageEventArgs e)
        {
            try
            {
                var routingOverrideDomainPart = "";
                var senderFullEmailAddress = e.MailItem.FromAddress.ToString().ToLower();	// get full sender email address
                var senderOnlyDomainPart = e.MailItem.FromAddress.DomainPart.ToLower();		// get domain part of sender email address
                var emailMessageId = e.MailItem.Message.MessageId.ToString();
                var emailMessageSubject = e.MailItem.Message.Subject.ToString();
                Microsoft.Exchange.Data.Mime.Header emailMessageOrgAuthAs;
                emailMessageOrgAuthAs = e.MailItem.Message.MimeDocument.RootPart.Headers.FindFirst("X-MS-Exchange-Organization-AuthAs");

                if (eventLogLevel >= 5) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Entering OnResolved for message " + emailMessageId + " (\"" + emailMessageSubject + "\").", EventLogEntryType.Information, 1);

                if (!_IgnoreAuthAsValues.Contains(emailMessageOrgAuthAs.Value.ToString().ToLower()))
                {
                    // X-MS-Exchange-Organization-AuthAs value not found in ignore table (typically "Internal") so we start processing the mail
                    if (!_RoutingTable.ContainsKey(senderOnlyDomainPart) && !_RoutingTable.ContainsKey(senderFullEmailAddress))
                    {
                        // Nothing to reroute, quit
                        if (eventLogLevel >= 5) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Exiting OnResolved for message " + emailMessageId + " (\"" + emailMessageSubject + "\"):\nSender email domain or address not found in rerouting table.\nSBR agent is not processing this mail.", EventLogEntryType.Information, 8);
                        return;
                    }

                    int rerouteCount = 0;

                    if (_RoutingTable.ContainsKey(senderOnlyDomainPart))
                    {
                        // sender email domain is in rerouting table
                        if (eventLogLevel >= 3) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Sender email domain " + senderOnlyDomainPart + " for message " + emailMessageId + " (\"" + emailMessageSubject + "\") is in rerouting table.", EventLogEntryType.Information, 2);
                        routingOverrideDomainPart = _RoutingTable[senderOnlyDomainPart];
                    }

                    if (_RoutingTable.ContainsKey(senderFullEmailAddress))
                    {
                        // sender email address is in rerouting table
                        if (eventLogLevel >= 3) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Sender email address " + senderFullEmailAddress + " for message " + emailMessageId + " (\"" + emailMessageSubject + "\") is in rerouting table.", EventLogEntryType.Information, 2);
                        routingOverrideDomainPart = _RoutingTable[senderFullEmailAddress];
                    }

                    var myRoutingOverride = new RoutingDomain(routingOverrideDomainPart);
                    var recipientsToOverride = e.MailItem
                                                .Recipients
                                                .Where(rec => !_InternalDomains.Contains(rec.Address.DomainPart.ToLower()));

                    foreach (EnvelopeRecipient recp in recipientsToOverride)
                    {
                        if (eventLogLevel >= 2) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Overriding recipient " + recp.Address.ToString() + " on " + emailMessageId + " (\"" + emailMessageSubject + "\").", EventLogEntryType.Information, 4);
                        RoutingOverride newRoute = new RoutingOverride(myRoutingOverride, DeliveryQueueDomain.UseOverrideDomain);
                        source.SetRoutingOverride(recp, newRoute);
                        rerouteCount++;
                    }

                    if (eventLogLevel >= 1) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Statistics for message " + emailMessageId + " (\"" + emailMessageSubject + "\"):\n" + rerouteCount.ToString() + " recipients  were rerouted.", EventLogEntryType.Information, 5);
                    if (eventLogLevel >= 4) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Exiting OnResolved for message " + emailMessageId + " (\"" + emailMessageSubject + "\").", EventLogEntryType.Information, 8);
                }
                else
                {
                    // X-MS-Exchange-Organization-AuthAs value found in ignore table
                    if (eventLogLevel >= 4) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Exiting OnResolved for message " + emailMessageId + " (\"" + emailMessageSubject + "\"):\nX-MS-Exchange-Organization-AuthAs header \"" + emailMessageOrgAuthAs.Value.ToString().ToLower() + "\" found in ignore table.\nSBR agent is not processing this mail.", EventLogEntryType.Information, 8);
                    return;
                }
            }
            catch (Exception except)
            {
                if (eventLogLevel >= 3) EventLog.WriteEntry("Microsoft.Exchange.SBR", "Exception (OnResolved): " + except.Message + "\n" + except.StackTrace, EventLogEntryType.Information, 10);
            }
        }
    }
}