Does anyone know of a tool/script for spliting large Unix mbox files up into smaller chunks ?

What I want is something that will take a large mbox file and split it into smaller ones based on some criteria (in most cases I probably want it split up by the year received, but in some cases I probably want it split down to the month level).

I'm sure there must be something like this out there already, though I haven't managed to find anything by Googling.