Exchange SE or the cloud?
The clock is at months, not years.

Exchange Server Subscription Edition (SE), released July 2025, is now the ONLY supported on-premises Exchange. It sits on the Modern Lifecycle (evergreen — no more version cliffs, support committed through at least 2035) and is code-identical to Exchange 2019 CU15 at RTM. The catch is commercial: SE requires a server licence plus CALs AND active Software Assurance or cloud subscription licences on all of them — on-prem Exchange is now effectively subscription-only.

No — and this is the decision hinge. Exchange 2019 (on CU14/CU15) upgrades to SE in place, like installing a cumulative update. Exchange 2016 cannot: its host OS caps at Windows Server 2016, SE requires WS2019/2022/2025, and upgrading the OS under Exchange is unsupported — so every 2016 estate needs a side-by-side ("legacy") migration to a new server. Microsoft also now advises migrating 2016 directly to SE rather than hopping via 2019.

Microsoft publishes no public prices for the SE server licence or CALs — pricing comes via resellers, which is why this calculator uses clearly-labelled editable estimates (indicatively, a Standard server licence in the high hundreds of dollars and Standard CALs around $100+/user, after the July 2025 rises of +10% on server products and +15/20% on the CAL suites). The mandatory SA/subscription is on top, and Outlook desktop is no longer included with any CAL. External users need no CALs. We quote real UK figures on request.

Often, but not always — which is why this tool does the arithmetic honestly. Exchange Online Plan 1 is $4/user/month, Plan 2 $8, Microsoft 365 Business Standard $14 (public US prices, July 2026; +5% if you pay an annual term monthly). Over three years cloud subscriptions can overtake SE's one-off licences for larger estates — but SE carries the mandatory subscription, hardware (for 2016 estates), patching and the CU treadmill. Small estates usually favour the cloud; data-sovereignty, integration and large-CAL estates can favour SE.

Yes, but it is nearly over. Microsoft ran ESU Period 1 (Oct 2025–Apr 2026) and Period 2 (May–end Oct 2026) — EA channel, per-server pricing via your account team, Critical/Important security updates only, no support cases. Microsoft has stated flatly there will be no further extension: after October 2026 there are no more updates for Exchange 2016/2019, full stop. From today that is a matter of weeks, not years.

Exchange SE CU1 lands H2 2026 and SE CU2 in H1 2027 — and CU2's installer will refuse to run while any Exchange 2013/2016/2019 server remains in the organisation. So whichever path you choose, all legacy Exchange must be decommissioned before you can take CU2 and stay current. Treat H1 2027 as the hard end of any coexistence/migration window.

Yes — this is live policy, not a threat. Exchange Online detects mail from persistently outdated on-premises Exchange servers and escalates: reporting, then SMTP throttling (450 4.7.230), then outright blocking (550 5.7.230) — reaching a 100% block after 90 days of enforcement. Any unsupported 2016/2019 server that emails Microsoft 365 tenants (i.e. most of the world) is on a countdown. Add Cyber Essentials — which fails unsupported, internet-facing software that cannot be de-scoped — and "do nothing" is the one option that does not exist.

Usually not any more. The free hybrid licence only ever covered recipient-management servers (no mailboxes, Edge or SMTP relay), and Microsoft's Exchange Management Tools now let you manage recipients with NO running Exchange server at all. If you do keep mailboxes or an SMTP relay on-prem, that server needs full SE licensing — which is exactly the hybrid trade-off the calculator prices.