I have set up my Yahoo Mail Plus! and for some reason it does not download my messages. I can however send new emails.
If change my password, then I get a bad password msg when I try to retrieve the msgs, but when my password is correct, it just says I have no new messages.
My provider is Sonera. I dont think there are any restrictions.
Have you made sure that your smtp settings are not Yahoo's but your service provider as this is the most common cause of peoples problems from what i have seen hope this helps and resolves your issues.