Update 6/13/2025: Please see our new blog post for updated timelines of releases as well as additional details on how to get from current versions of Exchange Server to Exchange Server SE: Upgrading ...
Updated Jun 13, 2025
Version 14.0
Blog Post
microsoft47 almmicrosoft MITHO-Exchange, after the end of support for Exchange Server 2019, will I still be able to communicate with Exchange Online, in an environment with a mailbox on Exchange Server 2019 and Exchange Online, or will I just not have support for updates, or will Exchange Server 2019 no longer communicate with Exchange Online with the release of Exchange Server SE?
Disclaimer; This is a wild guess :-)
Staying on Exchange Server 2019 after its end of support in October 2025 will place your environment in an unsupported state, particularly for hybrid deployments with Exchange Online. While hybrid functionality such as mail flow, mailbox migrations, and free/busy lookups will likely continue to function for a time, Microsoft will no longer provide security updates or support for Exchange 2019, including hybrid connectivity issues.
Historically, Microsoft has taken enforcement actions against outdated Exchange servers relaying to Exchange Online. For example, environments running unsupported Exchange versions or unpatched servers have faced throttling and eventual blocking of SMTP traffic to Exchange Online. It’s reasonable to assume a similar enforcement approach will apply to Exchange 2019 once Exchange SE reaches CU1 or CU2, and Microsoft starts pushing adoption of the new version.
It would be wise to plan for the move to Exchange SE ahead of the support deadline. Fortunately, Exchange SE supports in-place upgrades from Exchange 2019 CU15, which simplifies the process significantly provided the underlying operating system is supported. If you're running an older OS, it may be more prudent to deploy a new Windows Server 2025 instance for Exchange SE instead of in-place upgrading.
