I'm a fan of some aspects of GTD and the way to deal with mail in particular as it got me back in control of the constant inflow of mail to my mailbox for the first time in 20 years or so! But the fact that I had to switch between pages to get an overview of all my mail annoyed me, being an old (and avid) Outlook user.
Then I realized that this was in fact a Kanban limit. (Or a WIP limit to be precise, if we consider queued email to be constantly gnawing away at the back of our mind, stealing focus and causing stress)
Every time the number of email in your inbox reach your page setting (be it 25 if you are bold, or 50 if you are more like me), it's time to schedule a few minutes to process mail and empty the page. Or schedule an email-Pomodoro.
Off you go now, you have reached your mailban limit while reading this!