I contacted Samsung and they basically said it was Yahoo's issue and to contact them.
This seems to be issue only on Samsung email app, as browser and yahoo app no issuesSame thing starting today for me. I can sign into Aol, but not into Yahoo. Well, actually it IS working, because everytime I go to sign into my yahoo account, I get a notification on my web browser that my account was just signed into, using the Samsung mail app. I tried 10 times, and all 10 times, my email (on the web browser) shows that my account was just signed into. However, every time, my samsung mail app says "unable to validate account" and takes me back to the sign in step, and does not add the yahoo email to the list of available email accounts.