A quick update on my "Using Thunderbird to Get Things Done" post:
In "Managing email with Thunderbird", Chillifrost makes an excellent point:
Only recently I discovered the power of mail labels and views. In a rush, I deleted all my imap sub-folders and defined views over the root (inbox) folder. Only when I had finished this exercise did the importance of folders strike me. Folders are certainly not passe. Folders have all to do with efficiency. In databases, simply designing views over tables is not enough; it is also necessary to have well designed database schema. A similar analogy holds for views and folders. If every view is written over the inbox folder and it contains thousands of mail, then computing views will be very time-consuming.In fact, I've augmented my labeling approach to GTD (described here) with folders. The most obvious one is "Archive". By definition, this label will wind up catching almost everything eventually: "Action required" and "Wait" messages get changed to either "Delete" or "Archive" once they're complete -- and since you delete one of those, "Archive" is the only one left. So in fact, I do have an "Archive" folder that I periodically manually copy all the "Archive" labeled messages from my inbox into. For what it's worth, it's at 8,530 messages right now. Whether or not I need to break it up further remains to be seen -- but if I do, it will most likely be by calendar year -- there's still not much reason to try to sort more granularly.
I also have a couple low-priority mailing lists I subscribe to that I don't want to clutter my inbox, so I do have filters to shunt them off to their own folders.
One shortcoming of Thunderbird for all this: The filters seem to be intended only for incoming mail, not for mailbox maintenance. There's no way to create a filter that says "If the message is in the inbox, is more than 30 days old, and has the label 'archive', move it to the 'Archive' folder." (It's only lacking the "has the label" bits -- which makes sense if filters are only intended for incoming messages, since labels are by their nature applied post-receipt.)