Exchange Online Protection puts problem messages into quarantine if it suspects that they contain spam, malware, or a phishing attempt. Instead of using the Security and Compliance Center GUI to work with quarantined messages, you can analyze details of quarantined messages with PowerShell to create some basic statistics and find messages that should be released.
https://office365itpros.com/2020/08/12/analyze-quarantined-messages-powershell/
