Encryption is like a relationship -- both parties need to be on the same page for it to work. And Microsoft and Comcast are apparently not on Google's page.
Google began a campaign Tuesday to raise awareness around encryption, and in the process it reported that less than 1 percent of emails sent during May from Gmail to Comcast.net accounts were encrypted in transit.
For Microsoft's Hotmail service (now called Outlook.com), just over half of emails to and from Google were encrypted. Outlook.com users can enable encryption but, unlike with Gmail, it's not turned on by default.
Google's figures appear in a new section in its transparency report that aims to give people better information on the security of their email.
The use of encryption has gained added attention since last year's leaks about U.S. government surveillance, prompting more service and software providers to promise customers they'll keep their data safe.
Encryption is meant to scramble messages and other data so it can only be read by the sender and receiver. Google has been encrypting all Gmail messages by default since 2010.
But encryption only works when it's supported by email providers at both ends of an exchange. In the figures it released Tuesday, Google said between 40 percent and 50 percent of all emails sent between Gmail and other providers during May were not encrypted in transit.
Yahoo fared better than others. Ninety-nine percent of inbound messages from Yahoo to Gmail accounts were encrypted, while 100 percent of outbound messages were.
Google's numbers don't reveal the proportion of emails encrypted within each provider's own walls. So it's possible that all messages sent among Microsoft's own users were encrypted, for example.
Microsoft said late last year that it would be expanding encryption across its services, with plans to encrypt all of its key communications services by the end of 2014.
"As we've said on the official Microsoft blog, we've been working to implement increased encryption across Microsoft products and services and are currently rolling out [transport layer security] in Outlook.com," a Microsoft spokesman said Tuesday.
Comcast did not immediately respond to comment.
An even stronger form of security is provided by "end-to-end" encryption, which encrypts data from the moment it leaves the browser, and keeps it encrypted until the intended recipient decrypts it.
Initially it is releasing only the source code for an alpha version of the tool. It wants the security community to test it and make sure it's rock solid. After that, Google will make End to End available in the Chrome Web Store.