After you delete a Yahoo Mail account, the turnaround process for signing up with the same username again would not be available for at least 90 days, as Yahoo does not immediately release the address. Also, all folders in your account will be flushed and of course that includes all mail messages contained within them, so you would not be able to recover any of those messages, even after you signed up again with the same username.
To request deletion for your Yahoo Mail account, visit this link ..
https://edit.yahoo.com/config/delete_user.
After you request that your account be deleted, it will take up to 90 days for all of the account data associated with your mail account to be permanently deleted.