Explanation Zero-hour auto purge (ZAP) is an email protection feature in Office 365 that retroactively detects and neutralizes malicious phishing, spam, or malware messages that have already been delivered to Exchange Online mailboxes. ZAP is available with the default Exchange Online Protection (EOP) that's included with any Office 365 subscription that contains Exchange Online mailboxes. ZAP doesn't work in standalone EOP environments that protect on-premises Exchange mailboxes
https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/zero-hour-auto-purge