I haven't tried Opera M2-- I'll check it out. Might be a fun hackathon project to train a Bayesian filter on every folder and auto-suggest routing at least.
There was a big IBM Research study a few years ago that showed it's dramatically more efficient to search email versus categorizing messages into folders. Here's a link to the full paper: http://people.ucsc.edu/~swhittak/papers/chi2011_refinding_em...