I know it's a bit less than obvious, but they're not equivalent or an "either/or". A "Smart Mailbox Folder" allows you to put your Smart Mailboxes into folders for grouping purposes. For instance, you could have a Smart Mailbox that collects all your messages to and from a friend of yours named "Sam", one that collected message to and from "Frieda", etc. and collect them all in a Smart Mailbox Folder titled "Friends". It's strictly an organization thing, for your own convenience, sort of like putting playlists in folders in iTunes, if you're familiar with that. If you only have a couple of Smart Mailboxes, a Smart Mailbox Folder probably won't be of much help, but if you have a bunch of Smart Mailboxes, organizing them into folders might be useful.
Hope this explains things.
Regards.
Message was edited by: Dave Sawyer