def add_campaign_id(campaign_id)
      fail(Mailgun::ParameterError, 'Too many campaigns added to message.', campaign_id) if @counters[:attributes][:campaign_id] >= Mailgun::Chains::MAX_CAMPAIGN_IDS

      set_multi_complex('o:campaign', campaign_id)
      @counters[:attributes][:campaign_id] += 1
    end
def add_campaign_id(campaign_id)
      fail(Mailgun::ParameterError, 'Too many campaigns added to message.', campaign_id) if @counters[:attributes][:campaign_id] >= Mailgun::Chains::MAX_CAMPAIGN_IDS

      set_multi_complex('o:campaign', campaign_id)
      @counters[:attributes][:campaign_id] += 1
    end
Add campaign IDs to message. Limit of 3 per message.

 @param [String] campaign_id A defined campaign ID to add to the message.
 @return [void]