This was my paradigm through the last years and I'm always promoting it to my colleagues. What for should invest something in folder structures (representing only one of many possible criterias)? Reliable automation for this task find its limits when subject-lines are not written in in a formally complete an consistent manner. And of course since Googlemail preached "never delete" we all stick to some email hoarding attitute (better having it than needing it, especially for long running projects there are some arguments). This way I left a 90 GB Mailbox at the company I worked for before. :-)

But such unstructured bunch of emails is hard to manage, if any subset is to be handled over to another person or to be archived in some other environment. Furthermore size limitations for the inbox will force you to get rid of something earlier or later. And there is no fun in going through thousands of emails from years ago. Therefor I recently started to at least using a rough structure of folders for archiving (all mounted as sub-folders of Inbox and therefore searchable at a glance).

Some categories (ToDo, Waiting for result etc.) I'm realizing by categories in Outlook. This is quite handy to follow-up as search-queries for those categories and more can be pinned as virtual folders in Outlook.

However, I wish there were more tools in Outlook for categorizing emails while leaving them all in the same place. I'm not sure whether I ever will become an exemplary user of Inbox Zero or even reach this state.