I started using email folders about a year ago and it has proved to be pretty useful. I'd highly recommend using email folders to store important emails. Email folders essentially work as tags. So projects that are put on hold or need to be revisited can be easily accessible through email folders. Delete the folder once the tasks are complete. This way tasks don't slip. I've been able to recover lost thoughts, projects, excel files, etc.
I wouldn't use email folders as primary tool to store thoughts or projects. More as an extension to an application like OneNote or Evernote.
Libby listen
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