Exchange 2016/2019 support ends in October: Plan your Exchange Server SE migration now and avoid risks

Red alert for your IT security! On 14 October 2025, Microsoft will discontinue support for Exchange 2016 and 2019. For on-premises and hybrid environments, this means no more security updates. The risk of serious cyberattacks, data loss and system failures increases dramatically.

The successor version Exchange Server Subscription Edition (SE) will not be released until the second half of 2025 - so there is not much time to react.

In this article, you will learn why updating to Exchange Server SE is essential, what risks the suspension of updates entails,  what steps are required to migrate to Exchange Server SE and what alternatives such as Exchange Online and Delos Cloud are possible.

Deadline: The end of support for Exchange 2016 and 2019

On 14 October 2025, Microsoft will finally end extended support for Exchange Server 2016 and 2019.  This means that no more security updates, bug fixes or technical support will be available for these versions after this date. But why is running an outdated Exchange environment so problematic and what are the risks involved?

Microsoft Exchange servers are often targets for active threats, and recent years have shown the importance of regular updates. A few examples illustrate how dangerous vulnerabilities in Exchange environments can become:

• HAFNIUM attacks (2021): This large-scale attack on on-premises Exchange servers has affected thousands of organisations worldwide, causing data leaks, tampering and costly system outages.
• ProxyShell/ProxyLogon vulnerabilities (2021): Critical exploits that allowed cybercriminals to gain access to Exchange Server. These incidents emphasise how important it is to apply security updates immediately.
• NTLM Relay Attack vulnerability (2024): This vulnerability allowed attackers to read user credentials via a relay and take over the account.

As these examples show, the risk of attacks and data loss increases drastically without security updates - a scenario that no company can afford. The secure operation of an email server is now essential in all industries. The BSI published a report at the beginning of 2024, which revealed that more than 17,000 instances of Exchange servers in Germany were vulnerable to vulnerabilities at that time.

If your company is still using Exchange on-premises solutions (Exchange 2016 or Exchange 2019) - whether as a primary system or as a hybrid server - now is the right time to take action and develop a strategy for the switch.

What does the new Exchange Server Subscription Edition (SE) bring?

Microsoft is relying on a completely new concept for on-premises Exchange Server with the upcoming Exchange SE: the subscription model (Subscription Edition). Instead, the SE version will be continuously updated, including security updates and feature enhancements.

Core features of Exchange Server SE:

• Flexible licence model:  Costs become transparent and plannable via the subscription model. In addition to the previously required client access licences (CALs) and server licences, Exchange SE also requires an active subscription. This can either be a cloud subscription (e.g. Microsoft 365 E3 or E5) or an Exchange SE licence with Software Assurance (SA). Depending on the licence model previously used, additional costs of between 10 % - 20 % are therefore to be expected.
• Continuous updates: No more major version jumps - new features and security updates will be delivered as updates upon release. Microsoft explicitly points out that new functions will continue to be implemented in Exchange SE from CU1 and beyond. However, no major innovations have been announced for Exchange SE itself.
• Hybrid compatibility: Optimally tailored to integration with Microsoft 365. If you do not provide mailboxes in your on-premises environment, the Hybrid Configuration Wizard (HCW) also offers a licence for Exchange SE. In this case, a Software Assurance (SA) or Microsoft 365 E3 / E5 licence is also required for Exchange SE and above.
• Modern security standards: Support for protocols such as OAuth 2.0, TLS 1.3 and modern authentication.

The aim of Microsoft is to offer companies a stable and future-proof platform without the need for migrations between complete product generations. This means that you are now facing a major migration of your Exchange on-premises infrastructure for the last time. All future steps should then be possible via an update.

Challenges and time pressure during migration

Migration to Exchange Server SE presents companies with a number of challenges - primarily due to the tight time window:

• Release of Exchange SE: Microsoft has now announced the release of the new version for July 2025, which is very close to the end of support for Exchange 2016/2019 in October.
• Potential risks due to bugs: Historically, new Exchange versions often have significant bugs in the first few months, which are only fixed by subsequent updates. Relying on the SE version immediately after release could therefore be risky.
• Complexity of migration projects: Migrating an Exchange server is time-consuming. Planning, test phases and coordination must be taken into account - time that is severely limited by the tight time window.

Our recommendation: Don't wait for Exchange Server SE! The window of opportunity for a smooth transition is too tight, especially if unexpected issues arise. Taking action now is crucial to minimise risks and ensure the security of your IT.

Recommended procedure for the migration

We recommend the following steps for the migration to Exchange Server SE, building on Microsoft's recommendations:

1. Analysis of the current environment: First, you should check what requirements exist and whether the existing infrastructure is suitable for the transition.
2. Upgrade to Exchange 2019:  Companies that are still using Exchange 2016 or older versions should first upgrade to the current Exchange 2019 version. This is the only way to easily install Exchange Server SE from July via an in-place upgrade. Anyone still running Exchange Server 2016 at the time of release will have to carry out a complex migration to Exchange SE within a short period of time.
3. Preparing a hybrid environment: Like its predecessors, Exchange SE is also optimised for hybrid scenarios with Microsoft 365. So if you want to use Exchange Online in the future, you should start setting up a hybrid environment now. This can make the later transition to Exchange SE easier.
4. Continuous monitoring and test phase: You should set up mechanisms to test the new platform as early as possible before Exchange SE becomes available.

Planning a migration project requires resources, expertise and time - so we don't recommend putting it off until the last minute.

What alternatives are there?

Despite the benefits of Exchange Server SE, alternatives often make sense - especially if you want to take action now. Two common options are:

1. Migration to Exchange Online: Exchange Online offers a fully cloud-based solution that is regularly updated and ensures optimal security and scalability. For organisations with lower on-premises requirements, this could be the most secure and cost-effective solution.
2. Migration to alternative platforms such as Delos Cloud: Delos Cloud is a new sovereign cloud that enables public clients in Germany to use a secure and regulated cloud. The services offered there include Office 365 services (including Exchange Online) as well as Microsoft Azure services and, in future, other solutions from well-known software providers.
   
   Some sovereignty features are:

• Physically separated from Microsoft
• Customer data remains in Germany
• Operation by a German cloud service provider
• Functionality and self-sufficiency remain guaranteed in the event of a crisis
  
  Claranet supports you in evaluating both options and helps you to find out which solution best suits your IT strategy.

Conclusion: What matters now

The end of support for Exchange 2016 and 2019 is forcing companies to act quickly. Whether you are switching to Exchange Server SE, migrating to Exchange Online or looking for alternative solutions, the most important priority is to plan and implement before the time window gets too tight.

Secure your Exchange future beyond 2025

The end of support for Exchange 2016/2019 in October 2025 is fast approaching. Act now to avoid serious security risks and system instability and future-proof your email communications.

Benefit from Claranet's many years of experience in planning and implementing complex Exchange migrations. We will help you to master the challenges and find the optimal solution for you.

In our webinar "Exchange 2016 & 2019 adé: Your options and the time factor" you will gain essential knowledge: Recognise critical pitfalls, discover future-proof alternatives (Exchange SE, Online, Delos Cloud), develop a precise migration strategy and proactively secure your systems.

How to secure your IT environment:

Way 1: Consulting package: Professional migration  

Use our "Professional replacement of Exchange Server 2016 / 2019" consulting package.

We work with you to develop a strategy for switching to a modern platform such as Exchange SE or Exchange Online and plan your migration efficiently and individually.

Way 2: Personalised advice from experts

Do you have specific questions or need a quick assessment of your situation? Arrange a non-binding consultation with our specialists. We will analyse your requirements and develop tailored recommendations. 

Take action now for secure and future-proof email communication and avoid the risks of the end of support in 2025.