A year or two ago, these same firms were bemoaning that they figured between 25% and 95% (depending which firm you listened to) of all email was spam. There's something a bit fishy fishy fishy here. My email quantity has been increasing at about a linear rate over the last four years, no major blips or dips.
If I turn off spam filters on all my email programs and let them fill up for a day, I get a few hundred. Say I get 240 emails across the four accounts I use. That's a rough figure, but tends to be close to the lower end of the quantity. About ten (10) per account are real emails sent by real people, let's say 12 to get to a round figure that will divide nicely into that 240 figure. That's 15% of my mail is stuff that are real emails. I'd say another 24 to 36 (to stick with round figures) per account are what was briefly called bacn, i.e. emails sent by supplier listbots, security systems, jokes lists, etc. That's about 48 emails per account that I can say are not spam. Out of an average of 60 emails, that's at least another 20% of my emails that is, by definition, spam.
And I'm being generous indeed there - because the amount of solicited email (i.e. the stuff I want or have requested) stays about the same from day to day, while the maximum number of emails I receive generally goes much higher, some days double, very rarely lower than 150.
So I'm blessed if I can figure out where they got their figures from.