I had 4,238. In a fit of inspiration, I just made a directory called "old" and move it all there.

Surprisingly, I'm no longer afraid of my mail client. It's easier to diligently write procmail rules now as new mail comes in.

Of course, if you come back to me in a couple of years it could easily be that dovecot is serving directories with names like "old", "flippingold", "goddamitinboxisstuffed" and so on.