You are unable to release the emails that are in quarantine. Emails continue to be blocked after sending for rescan.
Email Security.cloud
If the message is clean on rescan, then it is delivered to its original recipient. If the message still contains malware or triggers a Scan-time Protection Policy rule, then it is quarantined again.
Possible rescan outcomes:
While releasing emails from Administrator Quarantine, the Release Emails dialog appears. You can specify whether to release them to the message’s original recipient, or to an alternate email address you specify.
Related article:
Releasing Legitimate Emails Quarantined by Anti‑Malware Service