private string GetAccountMessageDirectory(Account account)
        {
            var mailbox = EmailAddressParser.GetMailbox(account.Address);
            var domain  = EmailAddressParser.GetDomain(account.Address);

            return(Path.Combine(_dataDirectory, domain, mailbox));
        }
        public async Task <List <DeliveryResult> > DeliverAsync(Message message, List <Recipient> recipients)
        {
            if (message == null)
            {
                throw new ArgumentNullException(nameof(message));
            }
            if (message.Recipients == null)
            {
                throw new ArgumentNullException(nameof(message.Recipients));
            }

            var commaSeparatedRecipientList = string.Join(", ", recipients.Select(item => item.Address));

            _log.LogInfo(new LogEvent()
            {
                EventType = LogEventType.Application,
                LogLevel  = LogLevel.Info,
                Message   = $"Delivering message from {message.From} to {commaSeparatedRecipientList}",
                Protocol  = "SMTPD",
            });

            var recipientsByDomain =
                recipients.GroupBy(recipient => EmailAddressParser.GetDomain(recipient.Address)).Distinct().ToList();

            var result = new List <DeliveryResult>();

            foreach (var domainWithRecipients in recipientsByDomain)
            {
                var ipAddresses = await _dnsClient.ResolveMxIpAddressesAsync(domainWithRecipients.Key);

                var remainingRecipientsOnDomain = domainWithRecipients.Select(item => item.Address).ToList();

                foreach (var ipAddress in ipAddresses)
                {
                    var client = new TcpClient();
                    await client.ConnectAsync(ipAddress, 25);

                    var connection = new Connection(client, CancellationToken.None);

                    using (var messageStream = _messageRepository.GetMessageData(message))
                    {
                        var messageData = new MessageData()
                        {
                            From       = message.From,
                            Recipients = remainingRecipientsOnDomain,
                            Data       = messageStream
                        };



                        var clientSession = new SmtpClientSession(_log, new SmtpClientSessionConfiguration(), messageData);
                        await clientSession.HandleConnection(connection);

                        foreach (var deliveryResult in clientSession.DeliveryResult)
                        {
                            var matchingRecipient =
                                recipients.Single(recipient => string.Equals(recipient.Address, deliveryResult.Recipient,
                                                                             StringComparison.InvariantCultureIgnoreCase));

                            switch (deliveryResult.ReplyCodeSeverity)
                            {
                            case ReplyCodeSeverity.Positive:
                                // Delete the recipient right away, so that if there is a crash we don't end up sending to this recipient again.
                                await _messageRepository.DeleteRecipientAsync(matchingRecipient);

                                result.Add(deliveryResult);
                                remainingRecipientsOnDomain.Remove(deliveryResult.Recipient);

                                _log.LogInfo(new LogEvent()
                                {
                                    EventType = LogEventType.Application,
                                    LogLevel  = LogLevel.Info,
                                    Message   = $"Message delivery from {message.From} to {deliveryResult.Recipient} completed",
                                    Protocol  = "SMTPD",
                                });

                                break;

                            case ReplyCodeSeverity.PermanentNegative:
                                // Let this recipient be deleted after we've submitted bounce message. This is delayed so that we don't
                                // lose the information if there's a crash.
                                result.Add(deliveryResult);
                                remainingRecipientsOnDomain.Remove(deliveryResult.Recipient);

                                _log.LogInfo(new LogEvent()
                                {
                                    EventType = LogEventType.Application,
                                    LogLevel  = LogLevel.Info,
                                    Message   = $"Message delivery from {message.From} to {deliveryResult.Recipient} failed permanently: {deliveryResult.ResultMessage}",
                                    Protocol  = "SMTPD",
                                });

                                break;

                            case ReplyCodeSeverity.TransientNegative:
                                _log.LogInfo(new LogEvent()
                                {
                                    EventType = LogEventType.Application,
                                    LogLevel  = LogLevel.Info,
                                    Message   = $"Message delivery from {message.From} to {deliveryResult.Recipient} failed temporarily: {deliveryResult.ResultMessage}",
                                    Protocol  = "SMTPD",
                                });
                                break;
                            }
                        }
                    }

                    if (!remainingRecipientsOnDomain.Any())
                    {
                        // No more recipients remaining.
                        break;
                    }
                }
            }

            return(result);
        }