In order to get email notifications whenever someone posts into a category, you should visit that category and click the "circle" icon at the top right corner of the frame. Several options are presented for "watching" a category or tag.
See https://meta.discourse.org/t/watching-and-tracking-implementation-for-tags-and-categories/46987/23 for more info.
The default state is signified by an empty circle... where you only get a notification if someone mentions your @name
You use the same interface element (circle icon at the top right corner of the frame) to "mute" a conversation that you might have been watching previously.