The problem could be a setting in "auto archive", this can be found after
clicking on a folder
and then "Folder Properties"
there is a setting here.
On the folder properties window, click on the "AutoArchive" tab
and check the option to "Archive this folder using these settings:".
Select a desired time period during which these emails will be stored on the Unwanted Emails folder (for example, one week),
then select the option to "Permanently delete old items".
As a result, these unwanted emails will be automatically erased by Outlook, after whichever setting is in place for example "a week " " 10 days" "a month "since they were received. it is not recommended for you not to delete the emails right after you received email, because it's always good to take a brief look on these emails, in case a known sender sent you a message from an email address which is not listed on your Contacts Address Book.