What is Exchange Online?

Exchange Online is Microsoft's cloud-hosted email and calendaring service, and it's the backbone of Microsoft 365 communications. When your Outlook inbox loads, when a meeting invite arrives, when a Teams calendar event syncs to your phone — that's Exchange.

What Exchange Online provides

• Mailboxes — typically 50 or 100 GB per user, with a connected archive of equivalent size on higher tiers.
• Calendaring — shared calendars, scheduling assistant, room and equipment mailboxes, and resource booking.
• Contacts — personal and organisational address books.
• Shared mailboxes — for addresses like support@ or sales@ that multiple people read.
• Distribution lists and Microsoft 365 Groups — for emailing groups of people, with the modern Groups option also providing a shared SharePoint site and Teams team.
• Mail flow controls — rules, connectors, and transport policies that decide how mail moves into and out of the tenant.

Clients you can use

Exchange Online works with:

• Outlook on Windows and Mac (desktop)
• Outlook on the web (browser)
• Outlook mobile for iOS and Android
• Teams calendar, which is really an Exchange calendar surfaced inside Teams
• Native mail apps on macOS and iOS via the Microsoft 365 connector

Security and compliance

Exchange Online includes anti-spam, anti-malware, and basic phishing protection out of the box. Higher tiers add Microsoft Defender for Office 365 for safer links, safer attachments, and advanced anti-phishing. Mailbox content is searchable from Microsoft Purview for eDiscovery and can be governed with retention and sensitivity labels. Litigation hold and in-place archive cover long-term retention.

Exchange Online vs Exchange Server

Exchange Server is the on-premises product that you install on your own servers. Exchange Online is the cloud service. Hybrid Exchange deployments — where some mailboxes are on-premises and some are in the cloud — are common during migrations, but Microsoft is steadily pushing customers fully into Exchange Online and deprecating on-premises features that won't come back.