Automatically copy a message in Mail.app to a directory

If I drag an e-mail to a directory, Finder makes a copy, so that the message also remains in Mail.app. I would like to make this happen automagically when the mail matches certain criteria (for example, all messages from John.Doe@ISP.net). The criteria is easy, but the action not so much. The only thing on the action menu that seems like it might work is "Run Applescript" but I know nothing about Applescript. In the "good old days", I'd use procmail but I suppose that's considered a dinosaur now even in the Linux world.