Mail doesn't accept my yahoo e-mail account

My problem: since Lion I'm really happy that I can use my Yahoo mail account in the official Mail app. It works also fine, but a fiew weeks ago suddenly mail stoped loading mails. I logd out in in the settings of Mail and reloged in but now I've the problem that Mail doesn't accept my datas... anyone who knows something?

Thanks

P.S. my mail adress ends with @ymail.com