I used yahoo’s smtp server (smtp.mail.yahoo.it) a long time for sending mails through Thunderbird, but recently it stopped working. I always received error messages that my mail was not relayed, but didn’t get a specific error code.
So, after looking around for a long time, I had to change my smt configuratin to the following:
smtp server: smtp.mail.yahoo.com Port: 465 Secure Connection: SSL Username: Userid without @yahoo.it
And this works again.