This guide goes over how you(end-user) can release or block emails in quarantine for Microsoft Defender
When an email is held in quarantine it's because it contains malware/Links to malware or was flagged as spam (it contains enough information that matches was spammers would send.)
If a incoming email is flagged you will receive a quarantine report within 4 hours and given several options for what you would like to do depending on why it was quarantined.
For Malware: You can review the message or Request Release
Review message - bring up a message review scrreen with a breakdon of the email and why it was blocked. you also have the option to preview the message here.
Request Release- Send a n