Microsoft is tightening the mail path between on-premises Exchange and Exchange Online, with older Exchange Server 2016 and 2019 builds set to face throttling and blocking. The change matters for hosts, MSPs and businesses that still relay mail from customer-managed Exchange servers into Microsoft 365.

In a Sept. 2 Exchange Team Blog post, Microsoft said that starting in the second week of September 2026 it will raise the oldest allowed version for Exchange 2016 or Exchange 2019 servers connecting to Exchange Online over an inbound OnPremises connector. Servers will need to be at least on the last public update baseline, released in October 2025, to avoid enforcement.

Microsoft says the policy is part of its existing program to throttle and block mail from “persistently vulnerable” Exchange servers. The company also warned that the next baseline increase will require a version newer than any public update available for Exchange 2016/2019, effectively pushing affected operators toward Exchange Server Subscription Edition or Microsoft’s Extended Security Updates program if they want to keep the hybrid mail flow.

For hosting providers and managed-service teams, the operational takeaway is straightforward: inventory hybrid Exchange connectors now, confirm patch levels, and warn customers that mail delivery to Microsoft 365 tenants may degrade or fail if legacy servers are left below the new baseline. The Register separately reported the enforcement as Microsoft moving to bounce mail from outdated Exchange servers.

Source: Microsoft Exchange Team Blog; corroboration from The Register.