Exchange Server SE or Exchange Online: Should You Keep Mail On-Prem?
Fix it now
Exchange Server Subscription Edition, released on 1 July 2025, is not a buy-once product. Microsoft requires an active qualifying entitlement – Software Assurance on Exchange Server licences, or a qualifying Microsoft 365 subscription with on-premises server use rights – to install it, run it and stay current, and the CALs need the same coverage. Exchange Online needs no CALs at all.
- Move to Exchange Online if you already hold Microsoft 365 Business Standard, Business Premium, E3 or E5. Exchange Online is in those plans and you are otherwise paying twice.
- Keep mail on-premises for a data residency, sovereignty or isolated-network requirement you genuinely cannot satisfy with a cloud region.
- Budget Exchange SE as a running cost, not a purchase. It follows the Modern Lifecycle Policy, so staying current is the condition of staying supported.
- Size the server edition on databases, not users: Exchange SE Standard supports 1 to 5 mailbox databases and Enterprise supports 1 to 100.
- Buy a Standard CAL for every user, and add an Enterprise CAL on top only for the users who need the advanced features. The Enterprise CAL is additive, not a replacement.
- Act now if you are still on Exchange 2016 or 2019. Exchange Server 2019 left extended support in October 2025, so the choice is Exchange SE or Exchange Online, not staying put.
If you already pay for a Microsoft 365 suite, the answer is Exchange Online and you can stop. Below is what the on-premises entitlement actually requires, what each route costs you in effort rather than money, and the mailbox and archive limits per Exchange Online plan.
Why it happens
Exchange Server SE changed the shape of this decision rather than the arguments in it. Microsoft’s Exchange licensing FAQ states that Exchange Server SE requires an active qualifying entitlement to install, run and stay current: either active Software Assurance on Exchange Server licences, or a qualifying Microsoft 365 subscription such as E3 or E5 carrying on-premises server use rights through an Enterprise Agreement. The CALs must be covered the same way. There is no buy-once-and-forget perpetual route to a supported on-premises Exchange any more.
The lifecycle policy reinforces that. Exchange Server SE started on 1 July 2025 and follows the Modern Lifecycle Policy rather than the fixed mainstream-and-extended model, so there is no published end-of-support date to plan a decade around and no long tail of security-only updates. Staying supported means staying current, which in practice means a cumulative update cadence somebody has to own. That is a genuine operational commitment on a server that is, by definition, reachable from the internet.
Exchange Online moves the whole of that to Microsoft and charges per user per month, with no CALs of any kind. Microsoft’s licensing FAQ confirms it uses a subscription model with Kiosk, Plan 1 and Plan 2 options, that resource mailboxes including room and equipment mailboxes are provided free of charge, and that shared mailboxes do not require a subscription provided delegate access comes from licensed users. Exchange Online Protection is included rather than being a separate product you have to buy and run.
You already hold a Microsoft 365 suite
You have this one if A tenant with Business Standard, Business Premium, E3 or E5 licences, and an Exchange server still running in a cupboard.
- Exchange Online is included in those plans. Running an on-premises Exchange as well means paying for the mailbox twice and carrying the patching risk for free.
- Check the mailbox size you are entitled to before assuming the move is a downgrade: Exchange Online Plan 1 is 50 GB and Plan 2 is 100 GB, and Microsoft’s 2026 packaging update added 50 GB of email storage to the Business plans.
- Where a server is kept only for hybrid recipient management, ask about the hybrid arrangement before buying a full server licence for it.
There is a genuine residency or isolation requirement
You have this one if A regulator, contract or air-gapped network that puts mail data somewhere specific, or nowhere reachable.
- Exchange Server SE, with the entitlement kept current – Software Assurance on the server licences and CALs, or a qualifying Microsoft 365 subscription with on-premises rights.
- Test the requirement against Exchange Online’s region options first. A surprising number of residency requirements are satisfiable in the service and were never checked.
- Budget the operational cost honestly: hardware, storage, a tested restore, anti-spam and anti-malware, and a patch cadence that responds within days when a vulnerability lands.
You are still on Exchange 2016 or 2019
You have this one if A production mailbox server on a version that has passed its end of support.
- Exchange Server 2019 left mainstream support on 10 January 2024 and extended support in October 2025. There is no supported standstill option.
- The two supported destinations are Exchange Server SE and Exchange Online. Choose on residency and operational capacity, not on which migration looks shorter.
- Exchange Server SE RTM is code-equivalent to Exchange Server 2019 CU15, differing in licence agreement, product name and build number, which is what makes the upgrade path from 2019 a manageable one.
Full reference
The two routes, compared on what they actually cost you
| Dimension | Exchange Server SE on-premises | Exchange Online |
|---|---|---|
| Licensing model | Server licence plus CALs, with active SA or a qualifying Microsoft 365 subscription required | Per user per month |
| CALs | Standard CAL for every user, Enterprise CAL additive for advanced features | None required |
| Included in Microsoft 365 suites | No | Yes, in the Business and Enterprise suites |
| Lifecycle | Modern Lifecycle Policy: stay current to stay supported | Continuous |
| Where mail data lives | Your hardware, your data centre | Microsoft data centres |
| Patching | Yours, and urgent when a vulnerability lands | Microsoft’s |
| Hardware and storage | You buy, size and replace it | None |
| Backup and recovery | Your responsibility, including a tested restore | Retention and recovery features; back up separately if policy demands it |
| Anti-spam and anti-malware | Separate product or service | Exchange Online Protection included |
| Mailbox size | Limited by your storage and your policies | Fixed per plan |
| Resource and shared mailboxes | Yours to create | Room and equipment mailboxes free; shared mailboxes need no subscription |
| Suits | Residency mandates, isolated networks, applications that talk to Exchange directly | Most organisations |
Server editions and CALs, if you stay on-premises
Exchange Server SE keeps the familiar two-part structure. The server edition is chosen on mailbox database count: Standard supports one to five mailbox databases and is aimed at small to midsize organisations, while Enterprise supports one to a hundred and is aimed at larger ones. That is a storage and design decision, not a feature one, and it is independent of the CAL question.
On the access side, the Standard CAL is the base licence every user needs. The Enterprise CAL is additive: Microsoft’s own wording is that to enable Enterprise CAL features the user must be licensed with one Standard CAL plus one Enterprise CAL. What the Enterprise CAL adds is the advanced set – archiving, multi-mailbox search, In-Place Hold and information protection features. Licence it for the users who need those, not the whole organisation.
Exchange Online mailbox and archive limits
| Plan | User mailbox | Archive mailbox | Auto-expanding archiving |
|---|---|---|---|
| Exchange Online Kiosk | 2 GB | Not available; can be added through Exchange Online Archiving | No |
| Exchange Online Plan 1 | 50 GB | 50 GB | No |
| Exchange Online Plan 2 | 100 GB | 1.5 TB | Yes |
| Microsoft 365 Business Basic, Standard, Premium | 50 GB in the service description, plus 50 GB added by the 2026 packaging update | 50 GB on Basic and Standard, 1.5 TB on Premium | Premium only |
| Microsoft 365 E3 and E5 | 100 GB | 1.5 TB | Yes |
| Office 365 F3 | 2 GB | Not available | No |
Auto-expanding archiving is the row that decides more designs than mailbox size does. On the plans that have it, a user starts with 100 GB of archive storage and Microsoft adds more automatically as that fills, up to 1.5 TB including the Recoverable Items folder. On Business Basic and Business Standard the archive is a fixed 50 GB with no expansion, which is where organisations with long retention obligations discover they needed Business Premium or an enterprise plan.
What Exchange Online does not remove
Moving to the service removes the server, not the responsibility. Retention, legal hold and recovery features exist in the service, but if your policy requires an independent backup of mail data then that is still a product you buy and run. Identity remains yours: a compromised account in Exchange Online is exactly as damaging as one on-premises, which is why conditional access and multi-factor authentication belong in the migration plan rather than after it. And applications that talk directly to Exchange – scanners, line-of-business systems, anything relaying SMTP – need a plan of their own before the mailboxes move.
Licence Mobility, if you are considering a hoster
Exchange Server is one of the server applications eligible for Licence Mobility through Software Assurance, which permits deployment in an Authorised Mobility Partner’s data centre or on Azure. Two conditions matter: the licences need active Software Assurance, and the deployment has to be registered with Microsoft using the licence verification form within ten days. Confirm the provider is an Authorised Mobility Partner before you commit. Separately, the Flexible Virtualization Benefit allows subscription or Software Assurance licences to run with an Authorised Outsourcer – a provider that is neither a Listed Provider nor using one as its data centre provider.
When a licence is the actual fix
If your organisation has a genuine reason to keep mail on its own hardware, Exchange Server SE Standard is the current licence and the only supported on-premises Exchange to build on now that 2016 and 2019 have passed end of support. It needs a server licence sized on mailbox database count, a Standard CAL for every user with Enterprise CALs added only where the advanced features are used, and an active qualifying entitlement – Software Assurance or a qualifying Microsoft 365 subscription – kept current. Arco supplies Exchange Server SE server licences and both CAL types, and will tell you plainly when Exchange Online is the cheaper answer, which it is for most organisations and is close to certain if you already hold Microsoft 365 suites that include it.
Questions people ask about this
Can I buy Exchange Server SE outright and keep it for ten years?
No. Microsoft states that Exchange Server SE requires an active qualifying entitlement – Software Assurance on the licences, or a qualifying Microsoft 365 subscription with on-premises server use rights – to install, run and stay current, and the CALs need the same coverage. It also follows the Modern Lifecycle Policy, so staying current is the condition of staying supported.
Does Exchange Online need CALs?
No. Microsoft’s licensing FAQ states that Exchange Online does not require CALs and uses a subscription model with Kiosk, Plan 1 and Plan 2 options. Room and equipment mailboxes are provided free of charge, and shared mailboxes need no subscription of their own provided delegate access comes from licensed users.
Which Exchange SE server edition do I need?
Choose on mailbox database count rather than user count. Standard supports one to five mailbox databases and Enterprise supports one to a hundred. The CAL decision is separate: a Standard CAL for every user, with an additive Enterprise CAL only for those needing archiving, multi-mailbox search, In-Place Hold or information protection.
We are still on Exchange 2019 – how urgent is this?
Urgent. Exchange Server 2019 left mainstream support on 10 January 2024 and extended support in October 2025, so it receives no security updates. The two supported destinations are Exchange Server SE and Exchange Online. Exchange SE RTM is code-equivalent to Exchange Server 2019 CU15, which makes the on-premises upgrade path a short one if you take it.
Is a mailbox smaller in Exchange Online than on my own server?
It is fixed rather than smaller. Exchange Online Plan 1 gives 50 GB and Plan 2 gives 100 GB, with a 1.5 TB auto-expanding archive on Plan 2 and on the enterprise suites. On your own server the ceiling is your storage and your policy, which is not the same as unlimited – it just moves the cost from a subscription line to a disk purchase.
Can I run Exchange Server SE at a hosting provider?
Yes, through Licence Mobility through Software Assurance, which covers Exchange Server among the eligible server applications and permits deployment in an Authorised Mobility Partner’s data centre or on Azure. The licences need active Software Assurance and the deployment must be registered with Microsoft using the licence verification form within ten days.
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