Hi Dan , thanks for your question! Users receive automatically email notifications when there are new posts in categories and topics they are following.
Optionally, you can set a category to be automatically followed by new users. You'll find this setting in Admin > Categories > Category Settings > Notifications. This setting only applies to new users who join, but contact us to have us retroactively apply it to all users for you.
By default, users will receive notifications of new topics in the category, but this setting can be changed to include new replies as well. Contact us to have that set as the default for your account.