The above might be good answers. But my 2 cents worth in case it helps someone, or Apple or Yahoo working on this, etc ...
Sometime this morning my iPhone 6+ stopped being able to connect to Yahoo email. My iPad and "old" iPhone 4S are still fine. All are iOS9.
The iPhone 6+ keeps prompting for my Yahoo email password, and on entering it, the dialog errors out saying "email address mismatch".
Each time I try I get an email from Yahoo (on the other devices) saying that IOS is attempting to use "emailaddress.x.y", being the "real" email address, not the nice human alias - see next.
For interest, I login and use a Yahoo alias email address for everything as the "real" or native address is a cryptic alpha+number string that is the ADSL login and not really much use for humans.
I've logged into Yahoo and confirmed my account, login password, alias and all kinds of things are working fine.
The iPhone in question still gets emails via my icloud address, but not Yahoo. The other two devices work fine. And my emails at yahoo.com are fine and still being received.
I've found some Google hits from months ago saying to delete the account from the iPhone and re add it. I'm really not wanting to do that!