Till now SSL protection or HTTPS encryption is offered for Yahoo Mail only till you sign into service, once you signed into it it will be turned off automatically, back in January 2013 Yahoo offered SSL option for users, now they enabled it by default for all users.
HTTPS Now enabled by Default for All Yahoo Mail users
“As we promised back in October, we are now automatically encrypting all connections between our users and Yahoo Mail. Anytime you use Yahoo Mail – whether it’s on the web, mobile web, mobile apps, or via IMAP, POP or SMTP- it is 100% encrypted by default and protected with 2,048 bit certificates” Jef Bonforte, SVP of communication products said in a blog post.
” This encryption extends to your emails, attachments, contacts, as well as Calendar and Messenger in Mail.” he added.