Categorization of Email is fundamental to getting things done.
The original way that Outlook organized email was by putting it in a Folder (eg: 'Projects/Foo/Design'). If you wanted to find that same email in another folder (eg: 'Projects/Foo/Meetings') you copied the email to the second folder.
Advantages: * Has a home Disadvantages: * An email had only one location (which is correct in terms of location, but not in terms of use). * If you responded to FolderA's email, it would not get attached to FolderB's email, and vice versa, leading to a disjointed conversation with the potential of losing important information.
Then Google came along and focused on Tagging emails.
Outlook responded, but they didn't do it by adding Tags, instead they beefed up their Category mechanism, and Search Folder architecture (why copy a successful competitors design when you can slap together a different contorted one, ey?)
It works by putting in the leg work to get organised.