Microsoft to bounce mail from outdated Exchange servers

Microsoft is intensifying its efforts to improve the security of Exchange Server, warning that legacy on-premises systems will be barred from sending email to Exchange Online. Starting in the second week of September, Exchange Server 2016 and 2019 servers that forward email to Exchange Online via an inbound connector must be updated to the final public update baseline released in October 2025.

Microsoft stated that this update level was released nearly a year ago, and all organizations should have implemented it. Microsoft has been progressively raising its oldest acceptable version, but this update pushes the baseline to the final public update for Exchange 2016 and 2019. Organizations that depend on on-premises Exchange servers for hybrid deployments, applications, or other email-related systems may retain them.

However, throttling and blocking will only affect servers sending email to Exchange Online through an inbound OnPremises connector. Other delivery methods remain unaffected, and the policy does not necessarily cover all servers in an organization. While the policy may evolve in the future, Microsoft has historically imposed throttling and blocking on significantly outdated Exchange Server installations.

Administrators must weigh the urgency of applying security patches against the potential risks introduced by updates. Microsoft's announcement provides administrators with advanced notice of an upcoming change, which would have occurred regardless of the announcement. Principal Project Manager Nino Bilic emphasized that administrators must understand the importance of timely Exchange updates.