It’s that big? I would think 95% is automated spam. Look at your spam folder to see how much is blocked

In this case they're saying "created by", as opposed to "a copy was received". The latter would be weighted much more heavily towards automation.

In other words, I think a human-written message to 10,000 customers would be counted here as +1 to "written by people", whereas a "Your account X is overdue" generated for one customer would be +1 to the not-by-people.