Microsoft Starts Blocking Outdated Exchange Emails in September 2026
Microsoft has announced that it will begin throttling and eventually blocking emails from Exchange Server 2016 and 2019 systems that are not updated to the final public security update baseline released in October 2025. This change will start rolling out in the second week of September 2026 and primarily affects hybrid environments that send mail to Exchange Online through an OnPremises inbound connector.
Organizations running older Exchange builds may first experience delayed mail delivery as Exchange Online throttles message traffic. If administrators do not update their servers, Microsoft will then begin rejecting messages entirely. Throttled servers will see temporary SMTP errors, and blocked servers will receive permanent delivery failures that prevent emails from reaching Microsoft 365 recipients.
Microsoft has used transport-based enforcement for several years to pressure organizations to keep Exchange servers updated and reduce security risks associated with vulnerable deployments. The company periodically increases the minimum supported version allowed to communicate with Exchange Online, and the latest change raises that requirement to the final publicly available security updates for Exchange 2016 and Exchange 2019.
Organizations that remain on these products could eventually need either Extended Security Updates (ESU) or a migration to Exchange Server Subscription Edition (SE) to maintain uninterrupted mail flow with Exchange Online. Microsoft has warned that future minimum version requirements may exceed the last publicly available updates, and companies that plan to keep these servers running should evaluate enrolling in the ESU program or accelerating migration plans.