versus

150 billion emails each day

These numbers do indeed fail the sniff test. This would mean that the average age of a mailbox would only be ~120 days (150B/2.3B daily mails/user = ~65, 8000/65 user account size/dail user mails= ~120 days). That's a mere four months. Of course, this is only if people aren't regularly deleting mail, but it gives an idea of how far out of whack the estimates are.

Later on, it's mentioned that 70% of mail is spam. Going from that same 150B number, that would mean that the average user would get ~20 emails per day that weren't spam. This is the average user, whose mailbox is only 4 months old, and probably hasn't signed up to all that many services...