Getting rid of your deleted emails in Outlook
Most Windows users are aware that once an email is deleted from his or her Inbox or Sent Items, the message can still be recovered from the Deleted Items folder. The email will remain in the Deleted Items folder until a user manually deletes it or when exiting if user agrees to the commonly seen Outlook prompt to empty the Deleted Items folder.
Not everyone is aware that even after messages are deleted from the Deleted Items folder they can still be recovered. In Outlook 2016, if you select the Deleted Items folder and click on the Folder tab, in the 'Clean Up' section you'll see the option to 'Recover Deleted Items'. This option will be activated if you are using an Exchange server account, but not if you're working with a .pst file. The 'Recovered Deleted Items' feature is also available in many past versions of Outlook.
Emails will kept available for recovery by 14 days by default with a system using Exchange 2016 (see the documentation on this at this Microsoft web page). [Unlike email messages, calendar entries are retained by default up to 120 days. See the Microsoft documentation on this point here. ] The maximum length of time that deleted items can be retained in Exchange 2016 is 24,855 days (which works out to more than 68 years - let's hope hope the MS Office monopoly doesn't last that long.) - See the MS TechNet posting on this point here.