Exchange After 2025: End of Life, Outlook Uncertainty and a Credible Open Alternative

Plenty of organisations across the region still have Exchange 2016 or 2019 quietly running in a server room, so if yours is one of them, you’re not alone. But time is up. Microsoft ended support for both versions in October 2025, and the paid security update programme that gave some organisations a little breathing room ends on 31 October 2026.

Microsoft’s answer is Exchange Server Subscription Edition. It is, for all practical purposes, the same server you already know. The difference is that you now have to keep paying to keep it supported.

In our experience, though, the server is not the hard part. Outlook is. Microsoft’s new Outlook for Windows will not connect to an on-premises Exchange server at all, and the classic Outlook your users rely on has a support horizon of roughly 2029. So the real question is not just "which server?" but "which server, and which Outlook?"

In this post we walk through the dates, try to make sense of the Outlook mess, and look at why twenty years of open source alternatives have struggled to replace Exchange, and what has finally changed.

Exchange 2016 and 2019: the last safety net goes on 31 October

Here is how we got here. Exchange 2016 and 2019 reached end of support on 14 October 2025. Microsoft then offered a paid Extended Security Update (ESU) programme for organisations that were mid-migration, and when that ran out in April 2026 it offered one more round. That second round ends on 31 October 2026, and Microsoft has made it clear there will not be a third.

What does end of support actually mean? Your servers will keep running. Mail will keep flowing. But no more security patches, no bug fixes, no time zone updates and no help from Microsoft when something breaks. For a system that sits on the internet with ports 25 and 443 open, that is not a risk we would want to carry.

It is worth being clear about what the ESU programme was, too. It was never a real extension. You had to buy it through a Microsoft account team, be on Exchange 2016 CU23 or Exchange 2019 CU14/CU15, and even then Microsoft only promised critical and important fixes, if any were released at all. When it first launched, Microsoft’s announcement said it would not go past April 2026, adding "you do not need to ask." Then it did, once. Don’t count on it happening again. Microsoft’s supportability matrix has the official details.

Exchange Server SE: the same server, now on rent

Exchange Server Subscription Edition arrived in July 2025, and it is now the only on-premises Exchange Microsoft supports. Under the hood it is Exchange 2019 CU15 with a new name. The bigger change is in how you pay for it. What changes is the entitlement. To keep running SE you need active cover: server licences and CALs with Software Assurance, or a qualifying subscription. If your Exchange 2019 licences still have active Software Assurance, they count. If that cover has lapsed, they don’t. In return there is no fixed end-of-life date. SE stays supported as long as you keep paying and keep installing every cumulative update. Fall behind on either and you are unsupported again.

The roadmap has also been a moving target. Microsoft announced that the first cumulative update would remove Outlook Anywhere, one of the ways Outlook connects to Exchange. Customers pushed back, and the removal was postponed with no new date. Dedicated SE licence keys are now expected in a later update.

So SE answers the compliance question: is our mail server supported? It doesn’t really answer the strategic one: where is on-premises Exchange heading? And when you look at the client side, that second question gets uncomfortable quickly.

The Outlook problem: two apps, one name

This is the part that confuses almost everyone we talk to, and it is not their fault. "Outlook" now means two completely different Windows apps, and only one of them works with an on-premises Exchange server.

The first is classic Outlook, the desktop app businesses have used for over two decades. It talks to Exchange directly and handles everything people expect: shared mailboxes, delegates, shared calendars, PST files, add-ins and proper offline mode.

The second is the new Outlook for Windows. It is a rebuild based on Outlook on the web, and it works through Microsoft’s cloud. According to Microsoft’s own documentation, it supports Microsoft 365 and Outlook.com mailboxes properly, and everything else only over IMAP. On-premises Exchange, including Exchange SE, isn’t supported as an Exchange account. And yes, the free Outlook app that comes with Windows 11 is this new one.

To make it more confusing, a Microsoft 365 subscription includes both. Microsoft planned to make the new Outlook the default for enterprise users in April 2026, then pushed that back to March 2027, the second year-long delay. Even then, users can switch back to classic during this "opt-out" stage, and admins control the timing. It’s a nudge, not a forced cutover, but it tells you which way things are going.

Which Outlook works with on-premises Exchange, and for how long

Read the table from top to bottom and a pattern jumps out. The last Outlook with a published end date that still works with on-prem Exchange is Outlook 2024, and it runs out on 9 October 2029. Classic Outlook in Microsoft 365 has the same horizon, just worded as a minimum. Microsoft hasn’t said when classic will leave Microsoft 365 for good, and it hasn’t committed to giving the new Outlook real Exchange Server support.

In plain terms: if you run Exchange on-premises, your users’ desktop client has an end date of roughly 2029. Picking a server without a plan for the client is just kicking the can down the road.

Where Microsoft is heading

To be fair to Microsoft, it has never said it is killing Exchange Server. But if you line up its recent decisions, they all point the same way, and that way is Microsoft 365.

Start with its own advice. Microsoft’s guidance for Exchange 2016 and 2019 customers recommends moving to Exchange Online first, with Exchange SE as the fallback. Then look at the new Outlook, which was built around Exchange Online and reaches other mailboxes through Microsoft’s own sync service. Proper Exchange Server support for it has been planned for more than two years and still hasn’t arrived.

The protocols are moving too. Exchange Web Services (EWS) is being switched off in Exchange Online, starting in October 2026 and finishing in April 2027 (Exchange Server itself isn’t affected), and legacy Outlook for Mac stops working with Exchange Online the same month. Its replacement, the Graph API, only exists in Microsoft 365. Meanwhile, on-premises Exchange now needs ongoing Software Assurance or a subscription, so if you were hoping to buy your Exchange server once and simply own it, that train left a long time ago. And classic Outlook’s "at least 2029" is a floor, not a promise. Several analysts expect it to be retired in the early 2030s.

Put together, it feels like a slow squeeze. You can still keep mail on your own servers with Exchange SE, but you are paying Microsoft on an ongoing basis for a server that depends on a client it is gradually phasing out. For organisations with data residency rules, regulated workloads or a sovereignty policy, now is the sensible time to look at alternatives, while classic Outlook still makes the move easy.

Twenty years of alternatives, and the one problem none of them cracked

People have been trying to get off Exchange for as long as we have been in this business. Several open source platforms came close. Each one ran into trouble with its business model, its licensing, or the same stubborn technical wall: getting Outlook to behave exactly as if it were talking to Exchange.

Zimbra: from open source challenger to commercial platform

Zimbra arrived in 2005 and quickly became the open source alternative everyone talked about. Then it changed hands, a lot. Yahoo bought it for USD 350 million in 2007, sold it to VMware in 2010, VMware passed it to Telligent in 2013, and Synacor picked it up in 2015 for USD 24.5 million. Four owners in eight years, and more than 90 percent of its value gone along the way.

With each change, a little of the open source spirit went too. In 2020 Synacor announced that Zimbra 9 binaries would only go to paying Network Edition customers, calling it an "open core" model. Today Zimbra’s own FAQ says the binaries are no longer free to download and use. Community builds kept things going for a while, but even Zextras, long the community’s biggest supporter, declined to build Zimbra 10.

The pricing drifted too. The features business users care about most, including the Outlook connector, live in the commercial Network Edition. There’s no perpetual licence option, and some customers say the per-account price is not far off Microsoft’s. Once an alternative costs roughly what Exchange costs, still needs an add-in on every PC and is no longer truly open, the case for switching gets a lot weaker.

A roadmap that kept changing direction

What really wore down confidence, though, wasn’t any single decision. It was how often the direction changed. As people who have run Zimbra through most of these releases, we have lost count of the U-turns. A few examples:

• The desktop client came and went, twice. Development of Zimbra Desktop was stopped under VMware in 2013, restarted in 2014, and then ended for good by 2019.
• "Drive" meant three different things. The original Zimbra Drive was a Nextcloud and ownCloud integration. When a new Drive arrived in version 8.8.12 as a replacement for the old Briefcase, the original was renamed Open Drive and later deprecated. Even admins on the official forum said they were confused. Then, in Zimbra 10, Drive was replaced by Briefcase, the very thing it had been built to replace.
• Core modules were swapped out mid-version. The same 8.8.12 release deprecated Zimbra’s classic backup, storage management and mobile modules, with removal announced for the next release of 8.8.
• Two web interfaces, two futures. Zimbra 9 introduced a "modern" interface borrowed from its cloud product, Zimbra X, and nudged customers towards that cloud platform. Years later, Zimbra 10 still ships the Modern and Classic web clients side by side, and users still need training to switch. Open source support ended abruptly. Zimbra 8.8.15 Open Source Edition reached end of support on 31 December 2023, and unlike previous releases, there was no technical guidance period afterwards. Zimbra 9 followed a year later.
• Even the licence keys changed. Zimbra Daffodil 10.1 brought a new licence daemon and a new key format replacing the old one, and requires newer operating systems.

Behind all this sat another layer of change at the top. Synacor, Zimbra’s owner, was itself taken private by investment firm Centre Lane Partners. Meanwhile Zextras, which had built many of the add-on features Zimbra customers relied on, put its energy into its own platform, Carbonio.

None of these decisions is unreasonable on its own. Products evolve. But taken together, they made it hard for customers and partners to plan more than a release or two ahead. And in our experience, that uncertainty, more than any missing feature, is what slowly eroded Zimbra’s popularity. When a platform’s own roadmap feels unpredictable, organisations start hedging their bets, and the community that once made Zimbra the default open source choice gradually thinned out.

We want to be fair here. None of this makes Zimbra a bad platform. It is mature and capable, and we still run Zimbra servers for clients today. Where it struggles is when users expect Outlook to behave exactly as it does with Exchange.

Zarafa and Kopano: close, but not quite native

Zarafa, from the Netherlands, was designed from day one as an Exchange alternative, with an AGPL-licensed server. Outlook connected through a proprietary plugin installed on each PC, and the free edition only allowed three Outlook users. The plugin was dropped in 2016 and Zarafa itself stopped being maintained in 2017.

Its successor, Kopano, went fully open source. To its credit, Kopano was upfront about the limitation: in a 2017 FOSDEM talk, it explained that its server could not be a drop-in Exchange replacement because it did not implement Exchange’s wire protocol. Outlook support moved to ActiveSync, plus an add-in to cover the gaps.

Carbonio: a fresh start, without Outlook

Zextras eventually built its own platform, Carbonio, derived from Zimbra. The Community Edition is open source, but Outlook can only connect over IMAP, POP3 and SMTP.

ActiveSync isn’t included in the free edition.

Stalwart: the modern newcomer we really like

If Zimbra is the veteran, Stalwart is the newcomer that has the whole industry paying attention, and honestly, we’re fans. It’s written from scratch in Rust, a language designed to prevent whole classes of memory bugs that have plagued mail servers for decades. Everything ships as a single binary: SMTP, IMAP, POP3, JMAP, CalDAV, CardDAV, WebDAV and a built-in spam filter, with no separate stack of services to wire together. It’s open source under AGPLv3, has had an independent security audit, and part of its development was funded by the EU’s NGI programme through NLnet.

The efficiency is genuinely impressive. According to Stalwart’s own documentation, it idles at around 100 MB of RAM, and 1 GB is enough for a small team of 5 to 10 users. Compare that with mail stacks that need several gigabytes before the first user logs in. For hosting providers, branch offices or anyone paying for hardware and power, that matters. These are vendor figures, of course, and real needs depend on mailbox sizes, search indexing and load, so benchmark your own workload before you size anything. It scales the other way too, with clustering, multi-tenancy and pluggable storage from SQLite to PostgreSQL and S3.

Stalwart is also one of the strongest supporters of JMAP, the modern IETF standard for mail (RFC 8620 and RFC 8621). JMAP replaces IMAP’s chatty, decades-old command set with a clean JSON API over HTTP, with efficient sync and push built in. It’s faster and far kinder to mobile batteries and patchy connections, and it now covers calendars, contacts and file storage too. If you were designing mail from a blank sheet today, it would look a lot like JMAP.

The catch, and it’s a big one for most offices we work with, is Outlook. Outlook doesn’t speak JMAP, and it doesn’t support CalDAV or CardDAV natively either. Stalwart doesn’t implement Exchange’s MAPI or EWS protocols, so classic Outlook connects over IMAP and SMTP, which gets you mail but not shared calendars, delegation or the company address book. For teams on Thunderbird, Apple Mail, mobile apps or webmail, Stalwart is an excellent choice. For an Outlook-centric office, it isn’t there yet.

The elephant in the room: Outlook

Every one of these platforms hits the same wall. Business users don’t just read email in Outlook. They share calendars, check whether a colleague is free, open their manager’s mailbox as a delegate, book meeting rooms, and expect every name to resolve from the company address book. All of that depends on Exchange’s own protocols.

IMAP gets you mail, but no calendars or contacts. ActiveSync was designed for phones, not for desktop Outlook. Plugins work, but someone has to install them on every PC, keep them matched to the server and hope the next Outlook update doesn’t break them. For twenty years the open question was simple: could an open source server speak Outlook’s language itself?

grommunio: an open source server that speaks Exchange

This is where grommunio comes in. Instead of bolting a plugin onto Outlook, it does the hard work on the server: it implements Microsoft’s Exchange protocols itself. It is open source (AGPLv3), developed in Austria, and as far as Outlook is concerned, a grommunio server is an Exchange server.

A quick note on spelling: grommunio is written in lowercase throughout this article, even at the start of a sentence, because that is how the company writes its own name.

• Outlook just works, with no plugin. Classic Outlook connects using the same protocols it uses with Exchange. Users add an "Exchange" account, autodiscover fills in the rest, and shared calendars, delegates and the address book behave the way people expect.
• Macs and iPhones are covered too. EWS support has been complete since 2023, so Outlook for Mac and Apple Mail and Calendar work. And because grommunio runs EWS on its own servers, Microsoft switching it off in Exchange Online doesn’t affect you.
• Phones connect natively. iOS and Android use their built-in Exchange account through ActiveSync, with device management included.
• It covers the long tail of protocols. IMAP, POP3, SMTP, CalDAV, CardDAV and LDAP are all there, along with a web client and optional chat, video meetings and file sharing. The features page has the full list.
• It runs where you want it. On standard Linux, on-premises or in a private cloud, with multi-tenancy for service providers. Your mail stays in your jurisdiction, on hardware you control.
• The costs are predictable. It’s free for up to five users, and commercial subscriptions start at EUR 1.99 per user per month. Remember that keeping Outlook still means paying for Outlook licences.

We’d be doing you a disservice if we didn’t add one caveat. grommunio is at its best as a replacement for mail, calendars, contacts and tasks. The chat and meetings modules are younger and less polished. It also isn’t a feature-for-feature copy of Exchange, so test the things your users rely on, such as shared mailboxes, free/busy, recurring meetings and room booking, in a proper pilot. But for the core Exchange workload, it gives you something Microsoft no longer offers: Outlook-compatible mail on servers you control, without paying Microsoft for the server. It isn’t free of subscriptions, though. Beyond five users you pay grommunio a per-user subscription, and Outlook itself still needs a Microsoft licence. The difference is that you choose where the system runs, and the server cost is typically far lower.

What grommunio still doesn’t match

Since we’re recommending it, it’s only fair to be specific about the gaps. Exchange has had more than 25 years to build out its features, and grommunio is still catching up in places. Here’s where we’d tell any client to look closely.

• Room and resource booking. This is the one we hear about most. Exchange’s resource booking is very rich: rooms can auto-accept only for certain people, send everyone else to a delegate for approval, enforce booking windows and maximum durations, restrict bookings to working hours, and accept a recurring series even when a few dates clash. grommunio handles the basics, auto-accepting and declining room bookings, but this area is clearly still maturing. Its own release notes show steady fixes through 2026: treating tentative and out-of-office slots as conflicts in February, fixing duplicate entries from meeting updates in June, and only in August adding booking confirmations for auto-accepted requests, handling cancellations properly and allowing back-to-back bookings. If your organisation relies on complex room policies, test them in the pilot before anything else.

• Compliance and legal tools. Exchange has a built-in compliance stack: archive mailboxes, retention policies, litigation hold, journaling and eDiscovery searches. grommunio’s answer is a separate archiving product, grommunio Archive. That covers retention and search for many organisations, but it isn’t a like-for-like replacement for litigation hold or Exchange’s eDiscovery workflows. Regulated businesses should map their legal and audit requirements before they move.

• Microsoft ecosystem integrations. Anything that depends on Microsoft 365 services, such as Teams calendar integration, Bookings, or Outlook add-ins that talk to Microsoft Graph, won’t work against grommunio. Graph only exists in Microsoft’s cloud.

• The new Outlook. As covered above, the new Outlook for Windows only connects to grommunio over IMAP. That’s Microsoft’s decision, not a grommunio shortcoming, but it limits you to classic Outlook on Windows.

• Admin tooling and depth of EWS. Exchange admins are used to the Exchange Management Shell and a huge library of scripts. grommunio has its own admin web interface and command-line tools, and only added a PowerShell interface in 2026. Its EWS support also keeps expanding: room lists, delegate management and offline address book files have all arrived in the last year. That’s good progress, but it tells you the edges are still being filled in, so check any third-party app that relies on EWS.

To be fair, a couple of common assumptions are out of date. grommunio added modern authentication (OAuth2) for Outlook in 2025, and it can now put a shared mailbox on a phone over ActiveSync, something Exchange itself doesn’t offer. The gap is real, but it is narrowing quickly, and for most day-to-day mail, calendar and contacts work, users won’t notice it.

Zimbra, Stalwart and grommunio side by side

Because we work with these platforms, this is a comparison we get asked about a lot. The short version: Zimbra makes Outlook work by installing an add-in on every PC, Stalwart takes the open standards route and leaves Outlook on IMAP, and grommunio makes the server speak Outlook’s own language.

How the Zimbra Connector for Outlook (ZCO) works
Outlook speaks MAPI, and Zimbra doesn’t. So ZCO is installed on each desktop and acts as a translator, converting Outlook’s requests into something the Zimbra server understands. It creates its own "Zimbra" Outlook profile, keeps a local copy of the mailbox and address book, and syncs in the background. It is part of Zimbra’s paid Network Edition and is closed source.

How grommunio works
There is nothing to install on the PC. The server speaks the same protocols Exchange does, so Outlook connects exactly as it would to Exchange, with its normal account setup and cached mode.

How Stalwart works.
Stalwart bets on open standards rather than Microsoft’s protocols. Mail goes over IMAP or JMAP, calendars and contacts over CalDAV, CardDAV or JMAP. That’s a clean, modern design, and clients built for those standards get an excellent experience. Outlook just isn’t one of them.

In day-to-day terms, ZCO means a desktop rollout, an add-in to keep in step with every server upgrade, and one more thing to check when Outlook misbehaves. Stalwart is a joy to run, light on hardware and built on standards that will age well, but Outlook users only get mail. With grommunio, moving Outlook users feels much closer to moving them from one Exchange server to another.

If your organisation is moving away from Outlook anyway, towards Thunderbird, Apple Mail, mobile apps or webmail, we’d put Stalwart at the top of the list. If Outlook is staying, grommunio is the more practical choice today.

One honest caveat that applies to all three: none of them works with the new Outlook for Windows beyond IMAP. ZCO is an add-in for classic Outlook, grommunio relies on classic Outlook’s Exchange protocols, and Stalwart was never in Outlook’s world to begin with.

Whichever way you go, you still need a plan for the desktop client, and Stalwart’s standards-first approach may well be the better long-term bet once classic Outlook is gone.

Choosing the right path, and why we can help

There isn’t one right answer for everyone. Some organisations should stay with Microsoft. Some should move to Exchange Online. And some are better off on an open platform they control. Making that call well takes real hands-on experience with every option, not loyalty to one vendor.

That’s where we think IP Technics is in an unusual position. Since 2009 we’ve designed, deployed and supported mail and collaboration platforms for organisations across the UAE and GCC, and we work with all three of the platforms in this article every day.

Exchange
We know Exchange from the inside: how it works with Outlook, how its upgrades go, and how its licensing has changed. We understand what organisations gain and lose when they move away from it.

Zimbra
We still run a number of Zimbra servers in production today, including for customers with thousands of users and terabytes of mailbox data. We’ve lived through every change described above, from the Outlook connector to the end of free binaries and the licensing shifts. Running it at that scale has taught us exactly where it shines and where clients run into its limits.

grommunio
We’ve run grommunio for customers since 2025, and we build and load-test it on our own infrastructure, including native Outlook connectivity, authenticated mail submission and large-attachment stress tests. Our advice comes from real deployments, not datasheets.

Moving between platforms
We’ve migrated organisations between different mail systems, not just between versions of the same one. Moving mail, calendars, contacts and shared folders across platforms at terabyte scale, while people keep working, is where most projects come unstuck. It’s also where experience counts most.

Because we aren’t tied to any one platform, we can recommend what actually fits. Very roughly, this is how we tend to think about it:

A typical engagement starts with an assessment of your current mail platform, mailbox sizes, integrations and every Outlook version in use. From there we lay out the options with three- to five-year costs and risks, and make a recommendation. If an open platform makes sense, we run a pilot with real users, shared mailboxes, calendars and phones before anyone commits. Then we plan and carry out the migration so users keep working in Outlook, and we can run the platform for you afterwards.

The bottom line

If you run Exchange on-premises today, you have three realistic options. Move to Microsoft 365. Stay on Exchange SE and hope classic Outlook outlasts your plans. Or move to a platform you control.

A few years ago, that third option meant giving up proper Outlook support. That’s no longer true. For organisations that want their mail in their own jurisdiction, under their own security controls and at a predictable cost, it’s now a practical choice.

Just don’t leave it too late. The last Exchange 2016 and 2019 security updates end on 31 October 2026, Office 2021 support ends on 13 October 2026, and classic Outlook’s horizon is 2029. Planning now means you move on your schedule, not Microsoft’s.

If you’d like to talk it through, we’re happy to take a look at your setup and give you an honest view, whichever platform it points to. You can reach us at iptechnics.com.