Joomla notifies user by email of new article
I am a Joomla user and have a lot of experience.
I just did a quick search and I think "JMailAlerts" might be a good option. While I'm not sure if it will send category updates to users instantly, it uses Cron-Jobs to automatically send email updates daily, weekly, or monthly. In addition, users can choose which updates they want to receive.
See it here: http://extensions.joomla.org/extensions/extension/marketing/mailing-a-distribution-lists/j-mailalerts
Hope this helps. I personally haven't used this extension because I didn't have to do what you are looking for. But it's worth the test! Good luck!
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AcyMailing Business / Enterprise does exactly that, but with a paid version of the extension. This can be achieved with the Auto-Newsletter plugin .
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