This is mostly inspired on this article. The main difference is that I talk about sup instead of mutt, and use SpamAssassin together with bogofilter. Having all these utilities installed and the sources for sup configured, you only need to touch procmail and sup configuration files.

My ~/.procmailrc contains a lot of comments, but what it does is basicly the following: If the message is smaller than 250 KiB, it calls SpamAssassin with it. This will create the headers X-Spam-Level and X-Spam-Status in this message. Only messages smaller than 250KiB are considered because most spam isn't bigger than that, and considering big messages would overload SpamAssassin. Then, if SpamAssassin consider this message as spam, tell bogofilter about it. Next, call bogofilter, which will add the X-Bogosity header to the message.

Now we have to configure sup. We need to tell it to update bogofilter when we manually mark a message as spam. For this we can use ~/.sup/hooks/mark-as-spam.rb. We must consider two cases: the case where bogofilter thought it was not spam, and the case where bogofilter was not sure about its status. In the first case, we must tell it to forget that the message was not spam and to register it as spam now. In the second, just register it as spam.

Also, we don't want spam on inbox. So we can edit ~/.sup/hooks/before-add-message.rb to look for the headers generated by SpamAssassin and Bogofilter and remove the :inbox label. It's important to mark unsure messages with a label so that we don't forget to mark it as spam if it is, and as not spam if it's not.

So we need a way to mark messages as not spam. I included a new keybinding in ~/.sup/hooks/startup.rb. Unfortunatelly, I don't know how to include a new keybinding in sup without repeating all the keybindings for the mode I'm dealing with, so that's how I've made it. The command removes the :unsure label and mark it as non-spam. It also sets the undo command, unregistering the message in bogofilter. The case where the message was previously marked as spam, and not unsure, is also handled.

I'm sure there's a lot of room for improvement, specially in the Ruby part, but it seems to be working.