Microsoft Cracks Down on Outdated Exchange Servers, Blocks Email

Microsoft has announced that it will soon block email from outdated on-premises Exchange Server installations. Starting from the second week of September, servers running Exchange Server 2016 and 2019 must be patched to the final public update baseline released in October 2025 to send emails to Exchange Online.

This change only applies to servers that use an inbound OnPremises connector to send email to Exchange Online. Other delivery methods are unaffected.

Microsoft's Principal Project Manager Nino Bilic explained that this move is simply a matter of enforcing existing security policies, stating 'With or without this announcement, the change would have been made (just as it was made in the past, without any announcement)'. The company has emphasized that administrators must balance the need to apply security patches promptly against the risk of introducing problems.

Some administrators have expressed concern about the timing and potential impact of this change, but Microsoft maintains that it is necessary for maintaining Exchange Server security. The update level was released in October 2025, almost a year ago, and all organizations are expected to have updated to it by now.