Emails from specific senders may be set up to go to the trash or other folders. Check your settings to view or change where they're going. If you've set up filters, the settings may be routing some emails to the trash. You can review your filter settings in Yahoo Mail and edit or delete any incorrect filters.
Thanks a lot for your reply. Unfortunately I have no filters or settings in my mail inbox. What frightened me is that I have found 700 emails from many different senders moved to the trash.
The only connection amongst them is they were the first 700 emails I received since I created my account.
That's why I thought there was an automatic system to cancel.