An Administrator had a problem:
Our company policy clearly states all email can be monitored at any time. I have couple employees sending confidential information and/or attachments outside the domain & then constantly purging their emails via right-click 'Deleted Items' folder which shows "Recover Deleted Emails" option in Outlook on their workstation. I need to DISABLE their ability to permanently delete emails.
While you can disable Outlook commands using group policy or by editing the registry, you can't prevent users from logging into Outlook on the web and purging deleted items recovery (unless you remove web access from those users).
My recommendation is to configure the server to journal their outgoing mail or use Transport Rules to BCC their mail to another address for monitoring.
Older versions of Exchange add the rules using the Exchange Management Console, Organization Configuration, Hub Transport. Select the Hub Transport Rules tabs to add rules to BCC these user's outgoing email or, if you prefer to use Journaling, select the Journal tab.
Office 365 administrators would open the Exchange admin center then select Mail Flow to add a rule to BCC messages or Compliance Management to set up journaling.
There are third-party utilities and services that will archive all messages sent or received and content control utilities that will scan mail for confidential information.