The Deleted Items folder contains all Messages you deleted, or those your Protection Center deleted. It is located in the Folder List on the left side of your IncrediMail (marked in red).
As extra protection, this folder will save messages, just in case you deleted accidentally.
To retrieve the message, simply right-click on it, choose 'Move to Folder' and choose your Folder.
Please Note: Once a message is deleted from the Deleted Items, it cannot be retrieved.