Yahoo mail be default is without SSL. What this means is that it is less secure when using public networks because data is sent and received without any encryption.
To enable SSL in Yahoo mail :
1. Sign into Yahoo mail account and select “Mail options” by clicking on the gear icon on top right.
2. Enable the tick mark next to “Turn on SSL” and save the changes.
3. After refreshing and logging back in, Yahoo mail will now be SSL enabled.