Exchange Online is Microsoft’s hosted email service inside Microsoft 365. It provides mailbox hosting, mail flow, calendaring, and built-in security controls such as spam filtering, phishing defenses, quarantine, and transport rules. Because it sits at the center of business communication, it is a high-value target and a critical dependency for daily operations.
In cyber security, Exchange Online appears in two major ways: attackers try to abuse it, and defenders rely on it. Threat actors may steal credentials, hijack sessions, create malicious inbox rules, or use compromised accounts to send internal-looking mail. Defenders monitor message traces, audit logs, quarantine queues, and service health to spot abuse or outages. A service-side failure, such as a quarantine glitch, can disrupt mail delivery and look like an incident even when no attacker is involved.