My suggestion, based on what worked for me -- would be to choose a few key categories that work for you ... and go with those.
YES. Make your Outlook & Mail folders the same. From experience: trying to remember two different systems gets to be too difficult.
I've posted the exact categories elsewhere in this forum, but one of the things that ended up working for me was to identify the 5 or 6 categories of information that I always managed -- and then use that SAME structure for my paper filing system, Windows "my documents" and email.
So for example, I (we - since we do this on our server & even for company files) have a folder in My documents called:
In Email, all correspondance with a particular client is sorted under
On paper, we have "Client-Contact" folders (for paper purchase orders, etc received from clients, copies of reports sent to them etc.)
Repeating the same structure greatly simplifies the system and most importantly, reduces the stress from trying to find things!