Microsoft starts blocking mail from out-of-date Exchange servers this week

Microsoft's Exchange team published a short post on 2 September announcing that from the second week of September 2026 it will raise the oldest allowed version for Exchange Server 2016 and 2019 machines that connect to Exchange Online over an inbound connector of the OnPremises type. The new floor is at least the last available public update, released in October 2025. Anything older gets throttled, then blocked.

Microsoft's framing was blunt. "This update level was released almost a year ago," the post said, "and all organizations should have updated to it, since."

Who this actually hits

Not everyone with a Microsoft 365 tenant. The change applies to servers connecting to Exchange Online over an inbound connector of the OnPremises type, and Microsoft was explicit that it does not apply to servers sending mail in other ways, and does not currently apply to all servers in an organisation. It also said that might change in the future, which is worth filing away.

The businesses in the firing line are the ones that moved to Microsoft 365 years ago and kept one Exchange server running on site. That happens more often than people expect: the server stays for hybrid mailbox management, or because an application, a scanner or a booking system relays mail through it, and nobody has had a reason to touch it since. The question is not whether you use Exchange. It is whether that box is patched to October 2025 or later.

The change after this one is the harder one

Microsoft flagged what comes next in the same post. The next time it raises the oldest allowed version, the required update will be newer than any publicly available update for Exchange 2016 and 2019. At that point only customers enrolled in the Extended Security Update programme, and customers who have moved to Exchange Server Subscription Edition, will meet the bar. Microsoft estimates that is several months away. So this week is a patching job, and the one after it is a licensing and migration decision, better made in October with a plan than in February with mail already bouncing.

Why the announcement is unusual

Microsoft has throttled and blocked mail from persistently vulnerable Exchange servers for several years, raising the bar periodically. What it has not done before is say so in advance. "So far, these changes have been implemented silently," the post noted. Nino Bilic, a principal project manager at Microsoft, put it plainly in comments reported by The Register: "With or without this announcement, the change would have been made (just as it was made in the past, without any announcement)." Some administrators were unimpressed by the notice, which is fair, because an Exchange update can introduce problems of its own. That is an argument for scheduling the work, not skipping it.

What to check this week

Start with the question most small businesses cannot answer quickly: do we still have an Exchange server on site, and what depends on it? If the answer is yes, find its build number and compare it against the October 2025 update for your version. If it is behind, get the update scheduled with a backup and a rollback plan rather than applied on a Friday afternoon. Microsoft also publishes guidance on pausing throttling and blocking, which buys time for a server that cannot be patched immediately, and that is a reprieve rather than a fix.

Then ask the bigger question. If the server exists only to relay mail from a photocopier or an application, there is usually a supported way to send that mail through Microsoft 365 directly, and retiring the box removes a whole class of future deadline.