Frequent login failure on Yahoo Mail from GMail app I have added my Yahoo Mail account to Gmail app on my Android phone (Samsung Galaxy Note 3, latest Gmail version)

However, ~5 times out of 6, the app fails to log in to Yahoo (the username/pwd and other settings are clearly correct since it does succeed in connecting once in a while).

Server is android.smtp.mail.yahoo.com, port 465. SSL/TLS security

Error is "Username or password is incorrect".

What could be causing this and what can I do to fix?
I encountered the same issue. I didn't find a solution yet, but I'd like share my opinion on the possible cause of this issue. 

After I re-try the sign-in, the sign-in page shows an error indicating redirection loop or proxy loop error. 

Sometimes, I get error message saying that I'm trying to login from an unexpected country, and I have to verify the log-in with a SMS message. 

Some other cases, if I login the www.yahoo.com with the browser, with password and CAPTCHA, on the same connection, I re-try the login in gmail app, it works. 

Most of the time, in the gmail app, when I re-try the log-in, I just get a dumb error message that leaves no clue. 

My theory is that the yahoo server does a lousy job when it tries to balance between the user convenience and security. The server is too sensitive to "suspicious" login attempt, and didn't have a better way to offer CAPTCHA validation with the gmail login.