Yahoo mail error code 475 occurs when a user finds some unusual activity on their account. The Yahoo server will block the request to access the account of the user in a bid to secure the account. This protects your Yahoo account from future attacks and secures it from the server-side.
You just need to wait for a few hours and sign in again in your Yahoo account and then change the login password. Your account will be more secure when you change the login password. And you should stop sending emails in bulk for a few hours to le Yahoo understand that you are using your account fairly and not misusing it.
Also read - http://www.articlesfactory.com/articles/technology/is-yahoo-small-business-free.html