As Exchange Online administrator, you should review from time to time what is being put in the Exchange Online Protection quarantine.
Now, in addition of the usual Release and Release & Report to release inappropriately quarantined message, you can Delete it. If you use the Ctrl or Shift key you can delete multiple quarantined email at once.
Reminder the Quarantine can be accessed from the Security & Compliance admin portal under the Threat Management\Quarantine section (direct URL https://protection.office.com/#/quarantine)
This option can help you cleaning a little the quarantine
NOTE this Delete option is NOT available to end-user accessing their quarantine using the URL https://admin.protection.outlook.com/quarantine