Office 365 tries really hard to let you recover messages you've deleted, just in case you didn't actually want to delete them. Even after messages have been put in the Deleted Items folder, and that folder has been emptied, the messages are still recoverable.

Usually. But here's the exception: If you delete an entire folder of messages (using the Delete item on the menu that appears when you right-click (or ctrl-click on a Mac) on the folder), the messages in that folder are gone, gone, gone. 

How to Recover Messages After They've Been Deleted and the Deleted Items Folder Has Been Emptied

These messages are recoverable under the following conditions:

• The messages were NOT in a folder that was deleted using the Delete item on the menu that appears when you right-click (or ctrl-click on a Mac) on the folder.

AND

• The Deleted Items folder was emptied no more than 30 days ago.

1. Launch Outlook on the Web in your browser (http://outlook.cornell.edu).
2. In the navigation pane on the left, click the Deleted Items folder.
3. Select the message or messages you want to recover.
4. In the gray toolbar just below the red, Cornell-branded toolbar, click Restore.  The messages will be restored to their previous location, whether your Inbox or another folder.
5. Repeat steps 3 - 4 to recover additional items as desired.

If you are unable to recover calendar items from the Recover Items folder, there are no further recovery options. You will need to recreate any events that have been lost.