The efficient ways of handling emails is to do it a few times a day, and then either deal with them to the point where you can delete it, or turn it into a task. (I have employees who are masters of efficiency and lean and this is their thing, but I’ve seen the benchmarks on departments where it was implemented, and it works.)

I can see how you could use kanban to do this, but most offices already have dedicated systems for handling their tasks, and honestly, even outlook tasks are better at this than a kanbanboard.

Why would you need multiple lines for an email? And why would you keep you emails on your email account, where no one else can get it, client if it’s important enough to archive?

Best of luck though, maybe there is a usecase I’m not seeing.