restrictEmailDomain.php

add_filter('fluentform_validate_input_item_input_email', function ($error, $field, $formData, $fields, $form) {
    
     /*
     * add your accepted domains here
     * Other domains will be failed to submit the form
     */
    
    $targetFormId = 20;
    $acceptedDomains = ['gmail.com', 'yahoo.com'];
    $errorMessage = 'The provided email domain is not accepted';

    if ($form->id != $targetFormId) {
        return $error;
    }

    $fieldName = $field['name'];
    if (empty($formData[$fieldName])) {
        return $error;
    }

    $valueArray = explode('@', $formData[$fieldName]);
    $inputDomain = array_pop($valueArray);

    if (in_array($inputDomain, $acceptedDomains)) {
        return [$errorMessage];
    }
    return $error;

}, 10, 5);