Microsoft to throttle outdated Exchange servers communicating with Exchange Online

Microsoft is increasing restrictions on older Exchange 2016 and Exchange 2019 servers that communicate with Exchange Online. Starting in the second week of September 2026, servers that have not been updated to at least the October 2025 update level will face email delivery throttling and eventual blocking.

This policy specifically impacts organizations using hybrid infrastructures. Experts, including BlueMind's DACH Country Manager David Voisin, warn that migrating mailboxes to the cloud does not necessarily eliminate the need for on-premises Exchange components. Local servers often remain essential for recipient management, mail routing, SMTP relays, and connecting internal applications.

Because regular support for Exchange 2016 and 2019 ended on October 14, 2025, companies may face unplanned costs related to Extended Security Updates or the necessity of transitioning to newer editions, such as the Exchange Server Subscription Edition. Organizations are advised to audit their entire mail architecture to identify remaining local dependencies and evaluate the security and economic viability of their current systems.