Microsoft will reject email from Exchange Server 2016 and 2019 installations that do not meet the October 2025 baseline, affecting delivery to cloud-hosted mailboxes.
Microsoft has announced that it will bounce email sent from outdated Exchange Server 2016 and 2019 installations. Only those meeting the October 2025 baseline will be able to deliver messages to cloud-hosted mailboxes.
The move is intended to encourage administrators to keep their on-premises servers up to date with the latest cumulative updates and security fixes.
Why this matters: Organisations running older Exchange Server versions may face email delivery failures unless they apply the required updates.
What this means for you: If you manage an Exchange Server 2016 or 2019 deployment, ensure it is updated to the October 2025 baseline to avoid email being rejected.
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