Issue:

While trying to clear the inbox of old or unnecessary emails, they are immediately sent back into the inbox after deleted.

Fix:

This is likely due to the "Deleted Items" folder being full, so deleted emails are not accepted into the "Deleted Items" folder and are returned to the Inbox.

1. In Outlook, right-click on Deleted Items and click Empty Folder.

2. After the Deleted Items folder empties, re-try deleting unnecessary emails from the inbox.