Email is an essential tool in the business world; being able to find old messages and exchanges is one of the rewards of a well-managed email environment.
But how do you keep the mass of emails under some sort of control? Is it better to organise your email history into folders for each client/project/ topic, or should you just lump everything together. Surprisingly, the formless lump approach has benefits.
A 2011 study from IBM  showed that people who file their email history into folder structures took longer to find messages than those who relied on the search functions of their email client.
This seems counter-intuitive (at least to me); but consider these impacts:
- Folder structures have to be maintained. If you use a client or project structure, you have to create new folders each time you get a new client or start a new project.
- Because your clients don't understand your folder structure, they won't always keep emails to a single subject. Making the decision about which folder each email belongs to takes time (not much for each individual message, but significantly more in total).
- A large folder structure is harder to use - the navigation overhead (scrolling, expanding etc)
Folders still have a place, but fewer, broader categories (such as 'Work', 'Personal') might make life easier.
Benefits of Chaos
A single folder of many emails is hard to search visually which can offset the filing simplicity. The solution is to use the search functions in your email client.
Emails are naturally searchable - they always include meta information about senders, recipients, subject, date etc - using these as search and sort criteria can dramatically reduce the number of candidate messages (the ones you have to look at).
You still need to archive emails occasionally - for performance if nothing else (a really big email file takes longer to load and consumes more system memory).
You might let your email client do your archiving automatically - if you're brave. Otherwise moving old emails to archive folders is easier if you have fewer folders to begin with.
I used to use client/quarter or client/year folders (Client A - 2011 Q1, Client B - 2011) but have since moved to a much simpler yearly Work/Personal approach. I already see the benefits in reduced administration time (because it's easier to file emails).
Ultimately do whatever suits your work pattern, although there's still no excuse for 3,000 messages in your inbox (you know who you are...)
 The IBM study: Am I wasting my time organizing email? A study of email refinding.