Exchange SE CU3 Is the New Coexistence Deadline

Microsoft has updated its Exchange Server SE upgrade guidance. In an October 27, 2026 update to its upgrade post, the Exchange Team says setup in Exchange Server SE CU3 is the release that blocks coexistence with Exchange 2016 and Exchange 2019. Earlier guidance pointed to CU2. If you still run either version, CU3 is now the line to plan against.

What changed

Microsoft's release table now lists three cumulative updates for Exchange SE:

• Exchange SE CU1 (H2 2026): the first new features. Coexistence with Exchange 2016 and 2019 is still possible.
• Exchange SE CU2 (H1 2027): no change to coexistence. Exchange 2016 and 2019 can still sit in the same organization, though Microsoft no longer supports them.
• Exchange SE CU3 (H1 or H2 2027): CU3 setup blocks coexistence with Exchange 2013, 2016 and 2019. Only Exchange SE can remain.

Microsoft hasn't published an exact CU3 release date. Treat the window as a target that could move, not a reason to wait.

Why it matters

To install CU3 or later, every older Exchange server has to be decommissioned and removed from the organization first. Switching a server off isn't enough. If an Exchange 2016 server is still registered when CU3 ships, you can't take that update until the old server is gone. Exchange 2016 and 2019 have also been out of support since October 14, 2025, so they don't get regular security fixes in the meantime.

What to do now

• Exchange 2019 CU14 or CU15: do an in-place upgrade to Exchange SE. Microsoft says it works like installing a CU, including for servers in a Database Availability Group.
• Exchange 2019 CU13 or earlier: update to CU15 first, then upgrade in place.
• Exchange 2016: update to CU23 if you aren't there yet. Join new Exchange SE servers, move mailboxes and mail flow, then uninstall the 2016 servers before CU3.

Microsoft advises against adding new Exchange 2019 servers as a stepping stone, since 2019 is already out of support. Migrating straight to Exchange SE is the recommended path.

We covered the full timeline, Extended Security Updates and licensing in our Exchange 2016 and 2019 end-of-support guide.

Plan the decommission, not just the upgrade

The extra CU gives Exchange 2016 shops a little more room, but the hard part hasn't changed. Moving mailboxes, public folders, connectors and hybrid settings, then cleanly removing old servers, takes planning and testing. Starting now leaves time to do it in stages.

If you'd like help mapping your path to Exchange SE, talk to our team.

Source: Microsoft Exchange Team Blog, Upgrading your organization from current versions to Exchange Server SE (updated October 27, 2026).