There is a lot out there regarding this topic. The latest paper by Whittaker et. al. should be interesting for you: "Am I wasting my time organizing email? A study of email refinding" http://people.ucsc.edu/~swhittak/papers/chi2011_refinding_em...

Also interesting on this topic are these papers:

* Better to organize personal information by folders or by tags?: The devil is in the details

* Don't take my folders away!: organizing personal information to get things done

* Keystroke Level Analysis of Email Message Organization