I want to do something more than just normal mail -> folder rules; I want to put a layer in front of my exchange server that lets me, for example, rewrite subjects into a more readable form.

I get automated messages whose subject format makes them almost useless. Changing the emitting software is not going to happen, so I need to do something client side.

I'm interested in anything that would allow me to do this, either on a mac, or on an older linux distro. Preferably the latter, and preferably in a way that I can turn on and forget about.