The server can put messages into whatever folders it wants. Different folders can even put the same message into multiple meta-folders. The whitelist can be stored as a message (as basically a file) in a config folder. This is pretty straightforward IMAP.
Again I'm not arguing that IMAP isn't creaky and clumsy but let's not pretend that lack of power is the reason they didn't use it.
Again I'm not arguing that IMAP isn't creaky and clumsy but let's not pretend that lack of power is the reason they didn't use it.