Microsoft is beginning to sever outdated on-premises Exchange servers from cloud mail more sternly. From the second week of September 2026, Exchange Online will start to restrict, and thereafter block, messages from Exchange Server 2016 and 2019 unless the servers have been updated at least to the final public level of October 2025. For companies that have deferred updating their corporate mail for years, the warnings have, in effect, come to an end.