This has a bit of a "I can build Dropbox with a bunch of shell scripts over the weekend" vibe. I'm sure there's a bit more to it than that. How would you whitelist things in your regular IMAP client? Manually moving things to the IMAP Inbox and which then marks them as whitelisted somehow?
You could probably build that with a bunch of hacks but you'll not get a polished consumer friendly feature out of that. They are not in competition with people running mutt and a bunch of custom scripts, it's more of an alternative for people using Gmail with Gmail apps currently and don't even know what's IMAP.