I also have a Yahoo Plus account. FWIW, my settings are somewhat different:
Server Name: smtp.mail.yahoo.com
Incoming Mail server: pop.mail.yahoo.com
Outgoing SMPT server: smpt.mail.yahoo.com
Use default ports (25, 465, 587)
Use Secure Sockets Layer (SSL)
User Name: MyAccount
Tried changing my settings to match yours as well as a few other things with the yahoo account. Coulden't get it to work.
Next i got a Gmail account that was free. Works like a charm. anyone else using Yahoo may need to switch email providers.
Don't know why it works with Gmail and not Yahoo but there you go...