I run my e-mail mostly on Outlook as an off-line client, and every January 1 I clean out my miscellaneous in-box and sent items into a storage folder with the old year's name on it, so the front-line folders don't get too full.
This year, while the in box was empty, I did something else. Whenever I need to send an e-mail when I'm not at my home computer, I use the webmail interface, which (unless I ask to delete it) saves my sent e-mail in a folder. I had about five years of that, which together was using up about half of the measly 100MB storage space my vendor offers.
Time to download it all so that it could be stored for back-file purposes. I did that, and sorted it by recipient. That made it fairly easy to pick out the ones that belonged in special folders, e.g. I keep a separate folder for each submission for Tolkien Studies so that I can track what we did with it.
Then resorted the remainder by date so that they could go in the year files, and it's done. Frankly I could throw out most of this, but every once in a while ...