Outdated Exchange Servers Could Soon Lose the Ability to Deliver Emails
What starts as slower email delivery could eventually turn into rejected messages, leaving organizations little room to ignore aging Exchange deployments.
Key Takeaways:
- Exchange Online will begin throttling email from outdated Exchange 2016 and 2019 servers in September 2026.
- Organizations that do not update to the October 2025 security baseline could eventually face message rejection.
- Microsoft expects customers to move to Exchange Server Subscription Edition or Extended Security Updates as support requirements continue to tighten.
Microsoft has warned that Exchange Online will begin throttling and eventually blocking email 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 begin 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.
Why Microsoft is raising the baseline?
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
This update is more significant than previous baseline increases because Exchange Server 2016 and 2019 reached end of support on October 14, 2025. Microsoft has warned that future minimum version requirements may exceed the last publicly available updates. 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.
What IT administrators should do?
Organizations that still rely on Exchange Server 2016 or 2019 in hybrid environments should first verify that all servers used to send mail to Exchange Online are updated to at least the October 2025 security update baseline. Microsoft warns that servers running older builds may experience mail delays and, message rejections once enforcement begins. Administrators should review Exchange build versions, test updates in advance, and monitor mail flow for any throttling or delivery errors that could affect business communications.
Going forwards, businesses should not treat this update as a one-time requirement. Exchange 2016 and 2019 are already out of support, and Microsoft is expected to continue raising the minimum accepted version for Exchange Online connectivity. Companies that plan to keep these servers running should evaluate enrolling in the Extended Security Update (ESU) program, while others should accelerate migration plans to Exchange Server Subscription Edition (SE) or alternative supported platforms.
Rabia has a master's degree in Software Engineering and she has years of experience writing professionally about Microsoft products and other technologies. Rabia has also written for OnMSFT.com as well as Windows Report. She is always up to date on t...
