Well, because it would be reasonable to assume that 40% of a typical business user received emails are something that should be ignored.
e.g. 10% of all emails are critical, important or urgent, 30% of emails aren't critical but need a response, additional 20% don't need a response but must be understood, and 40% are just "organizational noise".
e.g. 10% of all emails are critical, important or urgent, 30% of emails aren't critical but need a response, additional 20% don't need a response but must be understood, and 40% are just "organizational noise".