You can overwrite this snippet by placing a custom version in site/snippets/email-manager/fields/checkbox.php.
Modify the CSS classes or structure to match your project’s requirements.
<?php foreach ($options as $key => $label): ?>
<div class="checkbox-option">
<input
type="checkbox"
id="<?= $key ?>"
name="<?= $field->name() ?>[]"
value="<?= $key ?>" <?= in_array($key, (array) $field->value()) ? 'checked' : '' ?>>
<label for="<?= $key ?>"><?= $label ?></label>
</div>
<?php endforeach; ?>