Microsoft to bounce mail from outdated Exchange servers

Microsoft announced that starting in the second week of September, it will prevent unpatched local Exchange Server 2016 and 2019 systems from sending emails to Exchange Online through OnPremises type inbound connectors. Affected systems must be updated to the final public update baseline released in October 2025; otherwise, their email sending functionality will be restricted or blocked. This policy applies only to servers that send emails through specific connectors and does not affect other delivery methods. Microsoft emphasized that this change will be implemented as soon as possible, and subsequent upgrades will only allow customers who participate in the Extended Security Update (ESU) program or have migrated to the subscription version of Exchange Server to meet the requirements.