> More than 300bn emails are said to be sent every day and if only one-third originate on Gmail—a conservative estimate—then a stack of print-outs would be 10,000km high.
I don't think that's a conservative estimate at all. In fact, I'd say it's a massive over-estimation.
Only one type of email is dominated by Gmail: personal email. I don't know about everyone, but I don't see many personal emails being sent.
Lots of those emails are for work, and there Gmail is not dominant at all. It's not insignificant, but Microsoft is a significantly larger part of the pile.
Another very large part of the total is automated emails, which are not sent through Gmail either.
Since Google Cloud doesn't include an email service, I'd go so far as to guess that more email originates from Amazon (SES) than from Google.