Outlook — Description of Deletion Methods
Microsoft Outlook has two basic types of item deletion:
A soft deletion occurs when simply pressing DEL on an item, or choosing Delete from the toolbar. In this situation, the item is merely moved to the Deleted Items folder.
A hard deletion occurs when pressing Shift-DEL on an item, or deleting an item already in the Deleted Items folder.
If the mail store is Microsoft Exchange (either on-premises or Exchange Online via Microsoft 365), an item that is hard-deleted then moves into the Recoverable Items folder. This is a hidden folder which is accessible via the Recover Deleted Items tool under the Folder tab. These items cannot be viewed until they are recovered (un-deleted) back to the main mailbox. After a period of time (depending on policy settings but likely 30 days), items in the Recoverable Items folder are purged.
An item that is moved from one folder to another may also appear in Recoverable Items, as the item was essentially hard-deleted from the original folder.