System.Net.Mail.SmtpTransport.SendMail C# (CSharp) Method

        internal MailWriter SendMail(MailAddress sender, MailAddressCollection recipients, string deliveryNotify,
            bool allowUnicode, out SmtpFailedRecipientException exception)
        {
            if (sender == null)
            {
                throw new ArgumentNullException(nameof(sender));
            }

            if (recipients == null)
            {
                throw new ArgumentNullException(nameof(recipients));
            }

            MailCommand.Send(_connection, SmtpCommands.Mail, sender, allowUnicode);
            _failedRecipientExceptions.Clear();

            exception = null;
            string response;
            foreach (MailAddress address in recipients)
            {
                string smtpAddress = address.GetSmtpAddress(allowUnicode);
                string to = smtpAddress + (_connection.DSNEnabled ? deliveryNotify : string.Empty);
                if (!RecipientCommand.Send(_connection, to, out response))
                {
                    _failedRecipientExceptions.Add(
                        new SmtpFailedRecipientException(_connection.Reader.StatusCode, smtpAddress, response));
                }
            }

            if (_failedRecipientExceptions.Count > 0)
            {
                if (_failedRecipientExceptions.Count == 1)
                {
                    exception = _failedRecipientExceptions[0];
                }
                else
                {
                    exception = new SmtpFailedRecipientsException(_failedRecipientExceptions, _failedRecipientExceptions.Count == recipients.Count);
                }

                if (_failedRecipientExceptions.Count == recipients.Count)
                {
                    exception.fatal = true;
                    throw exception;
                }
            }

            DataCommand.Send(_connection);
            return new MailWriter(_connection.GetClosableStream());
        }
    }

        public void Send(MailMessage message)
        {
            ObjectDisposedException.ThrowIf(_disposed, this);

            if (NetEventSource.Log.IsEnabled())
            {
                NetEventSource.Info(this, $"DeliveryMethod={DeliveryMethod}");
                NetEventSource.Associate(this, message);
            }

            SmtpFailedRecipientException?recipientException = null;

            if (InCall)
            {
                throw new InvalidOperationException(SR.net_inasync);
            }

            if (message == null)
            {
                throw new ArgumentNullException(nameof(message));
            }

            if (DeliveryMethod == SmtpDeliveryMethod.Network)
            {
                CheckHostAndPort();
            }

            MailAddressCollection recipients = new MailAddressCollection();

            if (message.From == null)
            {
                throw new InvalidOperationException(SR.SmtpFromRequired);
            }

            if (message.To != null)
            {
                foreach (MailAddress address in message.To)
                {
                    recipients.Add(address);
                }
            }
            if (message.Bcc != null)
            {
                foreach (MailAddress address in message.Bcc)
                {
                    recipients.Add(address);
                }
            }
            if (message.CC != null)
            {
                foreach (MailAddress address in message.CC)
                {
                    recipients.Add(address);
                }
            }

            if (recipients.Count == 0)
            {
                throw new InvalidOperationException(SR.SmtpRecipientRequired);
            }

            _transport.IdentityRequired = false;  // everything completes on the same thread.

            try
            {
                InCall    = true;
                _timedOut = false;
                _timer    = new Timer(new TimerCallback(TimeOutCallback), null, Timeout, Timeout);
                bool   allowUnicode    = false;
                string?pickupDirectory = PickupDirectoryLocation;

                MailWriter writer;
                switch (DeliveryMethod)
                {
                case SmtpDeliveryMethod.PickupDirectoryFromIis:
                    throw new NotSupportedException(SR.SmtpGetIisPickupDirectoryNotSupported);

                case SmtpDeliveryMethod.SpecifiedPickupDirectory:
                    if (EnableSsl)
                    {
                        throw new SmtpException(SR.SmtpPickupDirectoryDoesnotSupportSsl);
                    }

                    allowUnicode = IsUnicodeSupported();     // Determined by the DeliveryFormat parameter
                    ValidateUnicodeRequirement(message, recipients, allowUnicode);
                    writer = GetFileMailWriter(pickupDirectory);
                    break;

                case SmtpDeliveryMethod.Network:
                default:
                    GetConnection();
                    // Detected during GetConnection(), restrictable using the DeliveryFormat parameter
                    allowUnicode = IsUnicodeSupported();
                    ValidateUnicodeRequirement(message, recipients, allowUnicode);
                    writer = _transport.SendMail(message.Sender ?? message.From, recipients,
                                                 message.BuildDeliveryStatusNotificationString(), allowUnicode, out recipientException);
                    break;
                }
                _message = message;
                message.Send(writer, DeliveryMethod != SmtpDeliveryMethod.Network, allowUnicode);
                writer.Close();

                //throw if we couldn't send to any of the recipients
                if (DeliveryMethod == SmtpDeliveryMethod.Network && recipientException != null)
                {
                    throw recipientException;
                }
            }
            catch (Exception e)
            {
                if (NetEventSource.Log.IsEnabled())
                {
                    NetEventSource.Error(this, e);
                }

                if (e is SmtpFailedRecipientException && !((SmtpFailedRecipientException)e).fatal)
                {
                    throw;
                }

                Abort();
                if (_timedOut)
                {
                    throw new SmtpException(SR.net_timeout);
                }

                if (e is SecurityException ||
                    e is AuthenticationException ||
                    e is SmtpException)
                {
                    throw;
                }

                throw new SmtpException(SR.SmtpSendMailFailure, e);
            }
            finally
            {
                InCall = false;
                if (_timer != null)
                {
                    _timer.Dispose();
                }
            }
        }