>> 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.

Given: thickness of paper 0.0039 inches Lets say each email is at least a page

0.0039 inches * 300*10^9 pages to km = 29,718 (km)

Maybe if some emails are more than a page its not too far off.