ext_localconf.php

<?php

defined('TYPO3') || die();

(function () {
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addService(
        'mailhandler',
        'submitmail',
        'tx_mailhandler_submitmail',
        [
            'title' => 'Mailhandler',
            'description' => 'Mailhandling',
            'subtype' => '',
            'available' => true,
            'priority' => 60,
            'quality' => 80,
            'os' => '',
            'exec' => '',
            'className' => \ChriWo\Mailhandler\Service\MailService::class,
        ]
    );

    $iconRegistry = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Imaging\IconRegistry::class);
    $icons = [
        'pagetree-folder-contains-mailhandler' => 'Module/module_mail.png',
        'mailhandler-table-mail' => 'Table/table_mail.png'
    ];

    foreach ($icons as $identifier => $file) {
        $iconRegistry->registerIcon(
            $identifier,
            \TYPO3\CMS\Core\Imaging\IconProvider\BitmapIconProvider::class,
            ['source' => 'EXT:mailhandler/Resources/Public/Backend/Icon/' . $file]
        );
    }
})();