To make room for new messages, delete some of your current messages from your account. If you would like to save them for later viewing, download them before deleting them.

When you delete messages in Yahoo! Mail, they are moved to the trash folder. Messages in your trash folder do not count against your quota. However messages in your trash folder can be deleted at any time without warning. Once messages are emptied from the trash folder, they are permanently deleted from the Yahoo! Mail server and cannot be recovered.

Your address book entries don't count towards your storage limit.