Has she changed the password recently? Changes to the password through the webmail interface (if the email provider offers one) are not going to automatically register for the same account when its viewed through a mail client. Check the incoming server settings as well as the outgoing settings (by clicking on the dropdown menu and selecting Edit Server List) and reinput the passwords in the provided fields just to be safe. Also consider, if the account she is trying to access is a paid one, has her subscription ended? What you're describing may be expected if its a .Mac/MobileMe account which did not automatically renew and as such she will be unable to access messages in that account until the subscription is renewed/reactivated.