I've got filters set up to organize in Outlook by Category, and manually flag items based on priority. I've found that cross-referencing For Follow Up flags by

I've got filters set up to organize in Outlook by Category, and manually flag items based on priority. I've found that cross-referencing For Follow Up flags by Category is the best way to keep emails from falling through the cracks. For everything else, there's Xobni. I only use folders for stuff like serve emails. For my personal mail, I'm generally happy with my options in Gmail.