I emphasize this is not a feature request but open-ended thinking, so yahoo vs. outlook vs. X is far too much detail. FWIW, I would likely move my email activities to a platform that offered this tagging. Omea gave me the ability to tag most anything, in one repository — web links, email messages, RSS news feed items, contacts, notes, etc. It is unsupported (but open source) and the Outlook plug-in broke on my machine. I’m familiar with a number of solutions, like the Outlook rules facility. I moved to Yahoo and Otherinbox to get a "cloud" solution with some automated inbox processing. Finally, right now I prefer a web email solution over a local hard-disk solution. Maybe gmail will become the new Outlook — i.e., the email repository for which folks write add-ons and interfaces. Dunno. Don’t really care.