Both Yahoo and Google have promised their mail service users that their respective inboxes will gain an additional perspective of security. Right now, there are over 425 million Gmail users and approximately 273 million Yahoo Mail users.
In 2015, Yahoo Mail will be encrypted end-to-end, as well as compatible with the end-to-end encryption which Google is currently developing to have it applied to their Gmail service. Earlier this year, we learned that Yahoo has begun to encrypt webmail at the data center level, although in 2015, we ought to be able to see encrypted emails being sent between accounts.
The goal of webmail encryption is important enough to both Google and Yahoo that they are going to combine their resources to make this vision happen.