The Blocked Lists for Exchange Online specify the blocked senders, URLs, SHA-1 hash values, and SHA-256 hash values for your organization through the Threat Remediation API. Email messages that match any item in the lists will be automatically quarantined by Cloud App Security.
After the blocked lists are configured, a message Blocked Lists for Exchange Online enabled displays on the Advanced Threat Protection screen for Exchange Online policies. The blocked lists apply to all targets in the enabled Exchange Online policies and take precedence over the configurations in these policies.