By default, if you empty your Outlook trash, you have 14 days to recover it before it is gone forever.
All you have to do is right-click on deleted items, and then click on “Recover deleted items….”
Assuming there is anything recoverable, you will get a list of recently deleted email you can get back.
Notice the Purge link at the bottom of the dialog. If you purge your items, there will no way to recover the mail without restoring backups.