Microsoft has reminded customers that the Exchange Server 2016 and 2019 Extended Security Update program ends in October 2026, and the October Exchange security update release is labeled as ESU-only for those versions. After that, any on-premises Exchange 2016 or 2019 servers you still run stop receiving security patches. The paths Microsoft points to are Exchange Server Subscription Edition (an in-place upgrade for 2019, a move to new servers for 2016) or Exchange Online through Microsoft 365. If email is still on-premises, price a Microsoft 365 migration against the cost of upgrading and licensing Exchange SE, and confirm the details in your quote.
Source: Microsoft Tech Community — Exchange 2016 and 2019 ESU Program Ends in October 2026 ↗