Existing wp plugin to bring to mind

(if the developer is to make quickly – long partnership guarantee)

Forum link:

1. Function subscribe for email only works for logged in users. To implement it accessible to all

2. To block the insertion of external links, e-mails and other spam in the topics and replies.

3. The plugin will be updated in the future, when you update reset all
files. Appropriately you need to make the update
you spelled out the things was easy to re-implement.