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!