What Is Microsoft Exchange Server? Editions & Versions

This is a working reference on Microsoft Exchange Server — the same material our Exchange Server support team uses when assessing environments. It covers what Exchange Server is, how it works, on-premises versus Exchange Online versus hybrid, the current version picture after the October 2025 end-of-support cutoff, editions and licensing, and system requirements.

What is Microsoft Exchange Server

Microsoft Exchange Server is a mail and calendaring server that runs on Windows Server inside an organization's own infrastructure. It hosts mailboxes, processes and routes email, and provides shared calendars, contacts, tasks, and public folders to clients such as Outlook, Outlook on the web, and mobile mail apps. Exchange is tightly integrated with Active Directory: user accounts, mailbox attributes, permissions, and the server configuration itself are stored in AD, and an Exchange organization cannot exist without an AD forest. First released in 1996, it remains the standard on-premises mail platform in Windows environments; its cloud counterpart, Exchange Online, is the mail service behind Microsoft 365. The official product documentation is at learn.microsoft.com.

How Exchange Server works

Mailbox databases

Mailboxes live in mailbox databases — Extensible Storage Engine (ESE) database files hosted on Mailbox servers. Each database has its own transaction logs, can be mounted or dismounted independently, and can be replicated to other servers through a Database Availability Group (DAG), which provides automatic failover by keeping passive copies of each database on up to 16 servers. Database count per server is what separates the Standard and Enterprise editions, covered below.

Client access protocols

Clients reach their mailboxes over HTTPS through a set of protocols the server publishes: MAPI over HTTP is the primary Outlook desktop protocol, Outlook on the web (OWA) serves the browser client, Exchange ActiveSync serves mobile mail apps, and Exchange Web Services (EWS) plus, for Exchange Online, Microsoft Graph serve programmatic access and third-party integrations. Autodiscover lets clients locate their mailbox server and configure themselves from just an email address and password. IMAP and POP remain available but are disabled or de-emphasized in most deployments.

Transport pipeline

Mail flow runs through the transport pipeline: SMTP mail enters through Receive connectors, passes through the Front End Transport and Transport services where it is categorized, routed, and evaluated against transport rules, and is delivered to a local mailbox database or handed to a Send connector for external delivery. An optional Edge Transport role can sit in the perimeter network to handle internet mail flow before messages reach internal servers. The pipeline is documented in the mail flow documentation.

On-premises vs Exchange Online vs hybrid

The same mailbox functionality is delivered three ways. The trade-offs:

Hybrid connects an on-premises organization to Exchange Online with shared free/busy, cross-premises mail routing, and mailbox moves in either direction. Organizations that synchronize identities from AD with Microsoft Entra Connect must keep at least one Exchange server (or the Exchange management tools) even after moving all mailboxes to the cloud, because on-premises AD remains the source of authority for recipient attributes. A cost and feature comparison between the two deployment models is in our Exchange Online vs on-premises breakdown.

Current versions: only Exchange Server SE is supported

Support for Exchange Server 2016 and Exchange Server 2019 ended on October 14, 2025 — no more security updates, bug fixes, time zone updates, or technical support, per the end-of-support roadmap. Exchange 2013 ended in April 2023.

The only supported on-premises version is Exchange Server Subscription Edition (SE), released July 2025. SE is code-equivalent to Exchange 2019 CU15 at RTM and follows a continuous-update model — cumulative updates instead of new numbered versions. Exchange 2019 upgrades to SE in place; Exchange 2016 requires a side-by-side migration to new servers. Details on what changed and the upgrade paths are in our Exchange Server SE guide, and the supported-build list is on the Exchange Server supportability matrix.

Editions and licensing basics

Exchange Server SE keeps the licensing structure of prior versions, with a subscription requirement added:

• Server license — one per server instance, in two editions set by product key: Standard Edition (up to 5 mounted mailbox databases per server) and Enterprise Edition (up to 100). Edition affects only database count, not features. See Exchange Server editions and versions.
• Client Access Licenses (CALs) — a Standard CAL per user or device, plus an additive Enterprise CAL for features such as in-place hold and advanced compliance. CAL type and server edition mix freely.
• Active subscription requirement — SE is not sold as a stand-alone perpetual product. Licenses must be covered by active Software Assurance or an equivalent subscription, or the use rights can come from qualifying Microsoft 365 plans with on-premises rights. Microsoft's Exchange licensing FAQ covers the models.

A server without a product key runs as a trial edition, functionally Standard, for 180 days.

System requirements summary

Exchange Server SE shares requirements with Exchange 2019. From the supportability matrix and the system requirements documentation:

• Operating system: Windows Server 2019, 2022, or 2025 (Standard or Datacenter); Server Core is the recommended installation option
• Memory: 128 GB minimum recommended for the Mailbox role, 64 GB for Edge Transport
• Active Directory: supported forest and domain functional levels, with a schema extension applied at setup
• Coexistence: SE installs into organizations running Exchange 2019 CU14+ or Exchange 2016 CU23 during migration; later SE cumulative updates block coexistence with end-of-life versions

Who still runs Exchange on-premises

Despite the general shift to Exchange Online, on-premises Exchange persists in specific situations: regulatory or contractual data-residency requirements that mail stay in a defined jurisdiction or facility; air-gapped or restricted networks (defense, industrial, government) where cloud mail is not permitted; organizations with AD-synchronized identities that must keep a management server for recipient administration; hosting providers and enterprises with custom transport or integration requirements; and organizations mid-way through long hybrid migrations. Microsoft's stated direction is cloud-first, and SE exists to keep these deployments supportable rather than to grow the on-premises installed base.

Administering Exchange

Day-to-day administration happens in two tools: the Exchange admin center (EAC), a web console for recipients, mail flow, permissions, and organization settings, and the Exchange Management Shell, the PowerShell interface that exposes every operation the EAC covers plus everything it does not. A walkthrough of the console is in our Exchange admin center guide.

FAQ

What is Microsoft Exchange Server used for?

Hosting business email, calendars, contacts, and tasks on an organization's own servers, delivered to Outlook, web, and mobile clients, with mail routing, retention, and compliance handled on the same platform.

Is Exchange Server the same as Outlook?

No. Exchange is the server that stores mailboxes and routes mail; Outlook is a client that connects to it. Outlook can also connect to Exchange Online and other mail services.

Which Exchange Server versions are still supported?

Only Exchange Server Subscription Edition (SE). Exchange 2016 and 2019 reached end of support on October 14, 2025; 2013 and earlier ended before that.

What is the difference between Exchange Standard and Enterprise?

The server edition, set by product key, controls mounted database count: Standard allows 5 per server, Enterprise allows 100. Features are otherwise identical; CAL type is a separate purchase.