That's been kicked around for years. https://www.google.com/search?q=Login+to+server+smtp.mail.yahoo.com+%22failed%22&ie=utf-8&oe=utf-8
Yahoo isn't that great at this system and has tried to move SMTP to a Freemium status a few times. I think Yahoo is the temp bug.
Bob
Using Thunderbird in Firefox Yahoo web based email. I am able to receive without issue. But for some reason, from time to time, I am unable to send. I will get the following message: Connected to smtp.mail.yahoo.com and then it times out. With the following message: Login to server smtp.mail.yahoo.com "failed". I can by pass my "Thunderbird" and go to the Yahoo site and I am able to send messages without issue.
I saw some steps to take to change the "smtp" server stuff, but I am reluctant to do so, since in a few hours I can usually come back and all is well. Maybe Thunderbird has a temp bug.
Any suggestions? Windows 8.1, All updates, Firefox, Dell Computer.
Thanks Eddie

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic