Bayesian filter for email importance field?

Question

The problem with importance in email is that it's set by the sender. Which leads some people to just set everything high when sending, ettiquette be damned.

Usually I write a set of mail filters for important mail like "from my boss" or nagios alerts to trigger something. As usual, it feels like Google deployed the idea shortly after I thought about using a bayesian filter on the recieving side instead of writing triggers. So I'm wondering: is there a commonly used tool or guide to setting up bayesian filtering of important mail for mail not hosted by google? It seems like it'd just be a slightly tweaked version of the normal spam filters!