Folder contents are permanently deleted when you delete a folder offline in Outlook

When you delete a folder while you are working offline, you may receive the following error message:

If you delete a folder while you are working offline, the corresponding folder on the Microsoft Exchange Server is also deleted when you work online again. All items in the folder on the server are deleted, regardless of whether they have been synchronized to your Offline Folder file. You should remove folders only when you are online.