Microsoft Defender for Office 365 is Microsoft’s cloud email security service for protecting mailboxes and collaboration traffic from message-borne threats. It inspects inbound and outbound email, attachments, links, and related content to detect phishing, malware, credential theft, and other delivery-based attacks before they reach users.
It matters because email remains one of the most common entry points for compromise. In practice, defenders use it to quarantine suspicious messages, block malicious URLs, and apply policy controls across Microsoft 365. In newer AI-driven environments, the same inspection layer can also help spot hidden instructions inside email content before they are consumed by tools such as Microsoft 365 Copilot. That makes it part of both traditional email defense and emerging AI security, where untrusted text may be used to influence system behavior.