Google has published data from 47,354 domains that it exchanges email with, showing what fraction of the traffic is protected from the view of packet sniffers in transit with TLS encryption. I filtered and crunched the data in Excel into this pivot table to show summaries per top level domain (TLD), broken down by traffic region. It's an interesting snapshot of the state of the internet at this particular juncture in our history. Ecommerce started in 1996 with the first public release by Netscape of SSL v3, yet here we are in 2016 and some emails are still being sent in the clear. This is fast changing though with the revelations of Edward Snowden. Formats available: Excel working file with raw data; png image; 2 page PDF; google sheets.
Attribution CC BY Creative Commons License
The XLSX file, Image, and Google Doc on this page (How Safe Is Your Email? Gmail Transport Encryption by Country) are released free of charge.