/// <summary>
        /// Is called when RCPT command has completed.
        /// </summary>
        /// <param name="op">Asynchronous operation.</param>
        /// <exception cref="ArgumentNullException">Is raised when <b>op</b> is null reference.</exception>
        private void RcptCommandCompleted(SMTP_Client.RcptToAsyncOP op)
        {
            if (op == null)
            {
                throw new ArgumentNullException("op");
            }

            try{
                if (op.Error != null)
                {
                    Dispose(op.Error);
                }
                else
                {
                    // Start sending message.
                    SMTP_Client.SendMessageAsyncOP sendMsgOP = new SMTP_Client.SendMessageAsyncOP(m_pRelayItem.MessageStream, false);
                    sendMsgOP.CompletedAsync += delegate(object s, EventArgs <SMTP_Client.SendMessageAsyncOP> e){
                        MessageSendingCompleted(sendMsgOP);
                    };
                    if (!m_pSmtpClient.SendMessageAsync(sendMsgOP))
                    {
                        MessageSendingCompleted(sendMsgOP);
                    }
                }
            }
            catch (Exception x) {
                Dispose(x);
            }
        }