Yahoo will begin spy-proofing its email system with end-to-end encryption technology, following in the footsteps of Google.
If successful, the technology will expand encryption beyond the domain of technologists and hackers, and safeguard the content of everyday messages.
The measures will be introduced by the end of the year, and come as internet companies face increasing pressure to protect the content of their customers' communications following revelations last year by whistleblower Edward Snowden about US government surveillance.
Google announced a similar project in June.
Both providers will rely on a Google-developed browser plug-in that uses so-called PGP encryption to scramble messages, rendering them unreadable by anyone other than the sender and receiver.
But the metadata, such as the email addresses of the sender and receiver and the subject line, will not be encrypted.
Announcing the plan during a presentation at US computer security convention Black Hat, Yahoo's chief information security officer, Alex Stamos, said Google's Gmail and Yahoo Mail would be compatible with one another.