Steps for an end user to release quarantined email in Office 365:
1. Connect to the security and compliance center: https://security.microsoft.com/quarantine (to browse to it in the Security and Compliance Center, go to Threat Management > Review > Quarantine.)
2. Search for the email that needs to be released.
3. Check the message and select Release message.
Release email from spam notification email:
The quarantined email message can also be released from a notification email seen below. The email will be delivered to users with any email messages that are blocked. Currently they are delivered from quarantine@messaging.microsoft.com.
For details on setting up a quarantine email notifications see use user spam notifications to release and report quarantined messages.