Cha, there is no the Reply service. The only way to achieve this is to add a new role service fro the Mailbox plugin. Please check the files below to see how it works for the Send Private Message service:
1. Here you can see how the services are added: /ow_plugins/mailbox/install.php
$authorization = OW::getAuthorization();
$groupName = 'mailbox';
$authorization->addGroup($groupName, 0);
$authorization->addAction($groupName, 'read_message');
$authorization->addAction($groupName, 'send_message');
2. Here you can see how to use the added service: /ow_plugins/mailbox/classes/create_conversation_form.php
$isAuthorized = OW::getUser()->isAuthorized( 'mailbox', 'send_message' );
If you are not familiar with the code modifications, it would be better
for you to find a developer who can help you with it. Please visit our
Oxwall Market (http://
www.oxwall.org/market/specialists).