UpgradeData.php

<?php
/**
 * KiwiCommerce
 *
 * Do not edit or add to this file if you wish to upgrade to newer versions in the future.
 * If you wish to customise this module for your needs.
 * Please contact us https://kiwicommerce.co.uk/contacts.
 *
 * @category   KiwiCommerce
 * @package    KiwiCommerce_EnhancedSMTP
 * @copyright  Copyright (C) 2018 Kiwi Commerce Ltd (https://kiwicommerce.co.uk/)
 * @license    https://kiwicommerce.co.uk/magento2-extension-license/
 */
namespace KiwiCommerce\EnhancedSMTP\Setup;

use Magento\Framework\Setup\UpgradeDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

/**
 * Class UpgradeData
 * @package KiwiCommerce\EnhancedSMTP\Setup
 */
class UpgradeData implements UpgradeDataInterface
{
    /**
     * Module upgrade data code
     *
     * @param ModuleDataSetupInterface $setup
     * @param ModuleContextInterface $context
     */
    public function upgrade(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $installer = $setup;
        $installer->startSetup();

        $data = [
            'scope' => 'default',
            'scope_id' => 0,
            'path' => 'enhancedsmtp/custom/failed_count',
            'value' => 0,
        ];
        $setup->getConnection()
            ->insertOnDuplicate($setup->getTable('core_config_data'), $data);

        $installer->endSetup();
    }
}