Simple definition
Exchange Online is the Microsoft 365 cloud messaging service that hosts mailboxes and related mail-transport functions.
Technical definition
The service manages user, shared, and resource mailboxes together with message routing, connectors, mail flow rules, domains, and organization settings. It integrates with tenant identities and Microsoft 365 security and compliance capabilities.
How it works / role
An inbound or outbound message passes through Exchange Online transport components, where applicable rules, connectors, and controls are evaluated before delivery. Administrators manage objects and settings through the Exchange admin center and supported administration interfaces.
What is it used for?
Host business email, centralize mailboxes, and manage an organization’s message routing, rules, and mail access.
Practical example
An organization creates a shared support mailbox, applies a mail flow rule, and uses a connector to exchange mail with a controlled application or gateway.
Common issues
- The license or mailbox type is inappropriate
- A connector or mail flow rule causes unexpected routing
- A domain, alias, or permission is misconfigured
- A security or compliance policy blocks legitimate mail
Related terms
Shared Mailbox · Distribution List · Defender for Office 365
Key takeaway: When troubleshooting Exchange Online, separate mailbox problems from transport, identity, and policy problems.