def send_message
      rkey = 'recipient-variables'
      set_multi_simple rkey, JSON.generate(@recipient_variables)
      @message[rkey] = @message[rkey].first if @message.key?(rkey)

      response = @client.send_message(@domain, @message).to_h!
      message_id = response['id'].gsub(/\>|\</, '')
      @message_ids[message_id] = count_recipients
      reset_message
    end
def send_message
      rkey = 'recipient-variables'
      set_multi_simple rkey, JSON.generate(@recipient_variables)
      @message[rkey] = @message[rkey].first if @message.key?(rkey)

      response = @client.send_message(@domain, @message).to_h!
      message_id = response['id'].gsub(/\>|\</, '')
      @message_ids[message_id] = count_recipients
      reset_message
    end
This method initiates a batch send to the API. It formats the recipient
 variables, posts to the API, gathers the message IDs, then flushes that data
 to prepare for the next batch. This method implements the Mailgun Client, thus,
 an exception will be thrown if a communication error occurs.

 @return [Boolean]