my unwanted and old mails cannot be deleted

Do or read some of these things to help clear your error.
1.When you try to access an email message that has been moved or deleted. For example,
if you delete an email from your inbox and hit the back button on your browser, the email
will still show up in your inbox. Even though it looks like the email is there, it really isnt!
2. If you clicked on a legitimate email message that wasn't deleted or moved and you get
an Error Code every time, click Check Mail and then try to access the same email again.
3. You have several windows or tabs open in the same Yahoo! Mail account. The Error
Code is a temporary problem which should go away once you close the additional tabs
or windows.
4. You're running the browser-based Yahoo! Mail service, and another mail service (such
as Outlook or your mobile phone email service) tried to access your Yahoo! Mail server.
Try shutting down your other email services to see if this removes the Error Code.
5. If that didn`t help try Yahoo Customer Service to fix the error.
Contact Customer Care (reply needed)
Is your Mail not working right? Need help finding or doing something?
http://help.yahoo.com/l/us/yahoo/mail/yahoomail/forms_index.html