Exchange Server 2016/2019 On-Premises to Exchange Online

Microsoft will continue to support on-premises Exchange via a new edition due to be released in Q3 2025. This coincides with the end of normal support for the current versions: 

• Exchange Version: Exchange 2013 
• Mainstream Support End Date: 10th April 2018  
• Extended Support End Date: 11th April 2023
   
• Exchange Version: Exchange 2016 
• Mainstream Support End Date: 13th October 2020 
• Extended Support End Date: 14th October 2025 
   
• Exchange Version: Exchange 2019 
• Mainstream Support End Date: 9th January 2024 
• Extended Support End Date: 14th October 2025 
  

Note: Only Exchange 2016 CU23 and Exchange 2019 CU15 are supported until 14th October 2025, all previous Cumulative Update versions are unsupported. 

Exchange Server Subscription Edition (SE) is due to be released early Q3 2025, with Exchange Server SE CU1 scheduled for late Q4 2025. Once Exchange Server SE CU1 is released, all previous versions will be unsupported. This applies to organisations running Exchange Server fully on-premises, in a hybrid configuration or even those only using the management tools for recipient management. 

While Microsoft are releasing Exchange Server SE to continue supporting organisations with Exchange on-premises, it is primarily aimed at large enterprise organisations. Minimum supported hardware specifications are very high, discouraging smaller-scale deployments, and version interoperability will be removed shortly after launch.  

Unless your organisation is already running the very latest version of Exchange, it will require significant effort to remain in a supported configuration; it could include hardware procurement, major version transitions and non-trivial configuration changes. The cost of this project as well as the effort involved would likely be high and exacerbated by the compressed timescales. 

The speed of a migration to Exchange Online can vary based on multiple factors, including the performance of the on-premises Exchange servers, network and internet link speeds/capacity, as well as the Microsoft cloud infrastructure itself. Is it also entirely dependent on the number and size of mailboxes in your environment. 

Migrations using the native Exchange tools typically provide the fastest and most seamless end-user experience. Mailboxes are pre-seeded in batches to Exchange Online in the background; the length of this part of the process is dependent on the factors above. Once all mailboxes in the batch are fully synchronised, they can all be cutover to Exchange Online in a matter of minutes with minimal disruption; provided Outlook clients are configured correctly, a restart of the client is all that is required to connect to their new Exchange Online mailbox.  

This process is consistent and will deliver good throughput for average mailboxes; Microsoft have observed up to 100GB per hour with multiple migration servers. As part of planning, Nasstar will help you to identify mailboxes that may take longer or require special consideration. These will typically be very large primary/archive mailboxes, or mailboxes with a very large number of messages; small mailboxes with lots of messages are slower to migrate than large mailboxes with fewer messages. 

Yes, all these items in your mailbox will be migrated to Exchange Online, however you should be aware that there is no guarantee that 100% of your data will migrate successfully; this is due to your on-premises Exchange platform. There is always a risk that some items may already be corrupted and there is no easy way to check for them, but migration reports will show these as failed items. There is unfortunately no way to recover these messages, but typically the percentage of these items over the course of a full migration is low. 

Exchange Online is generally considered to be more secure than Exchange On-Premises. Microsoft heavily invests in security of the Microsoft cloud platform to protect your data and your users accessing it. 

• Modern authentication: While Exchange on-premises can be configured to support Modern Authentication, Exchange Online uses it by default. It provides significantly stronger authentication through standard protocols and enables features including Multi-Factor Authentication (MFA), passwordless logon and biometrics. 

• Conditional access: The use of Modern Authentication also enables Microsoft Entra ID Conditional Access, which grants access to Exchange Online (and other cloud services) based on criteria that you define.  

• Legacy protocols blocked by default: While you can still use POP3, IMAP4. and SMTP AUTH in Exchange Online, these now require modern authentication; legacy authentication for these protocols is now blocked. At time of writing, basic authentication with SMTP AUTH is still possible, however this is due to be removed in September 2025. 

• Security updates: Microsoft manage the infrastructure required to run Exchange Online, and as a result they maintain the underlying operating system and application updates/patches. You don’t need to worry about update/patching windows or outages and can be assured that any security vulnerabilities are fixed quickly and with no disruption to the service. 

If a migration is properly planned and scheduled, users may not even notice that their mailbox has been moved to Exchange Online (although obviously we should tell them!). The native migration process is designed to be almost seamless, and the pre-seeding and cutover operations mentioned earlier are what makes this possible.