Exchange Online Blocks Legacy Senders

• The Exchange Online cloud service progressively throttles, then blocks, mail connections with “persistently vulnerable” Exchange Server hosts.
• The policy can disrupt e-mail for Exchange Server customers and the Exchange Online customers they communicate with.
• To avoid blocking, customers on Exchange Server 2016 and 2019 must upgrade to Exchange Server SE, migrate to Exchange Online, or get off Exchange completely by Oct. 14, 2025.

Exchange Online progressively blocks e-mail exchanges (SMTP connections) with what Microsoft calls “persistently vulnerable servers”, Exchange Servers that do not have the latest security updates. The policy is not new, but on Oct. 14, 2025, all Exchange Server 2016 and Exchange Server 2019 hosts will become persistently vulnerable, because they will not be eligible for security updates after that date. That means they may be cut off from Exchange Online at any time with 90 days’ notice.

Exchange Online blocks a persistently vulnerable Exchange Server host progressively over 90 days. (See the accompanying chart.) To the Exchange Server host, Exchange Online appears down for an increasing fraction of each hour, until the host can no longer connect. Both sides of the connection will receive distinctive warnings and errors, so administrators know that the connection is being blocked rather than (for example) being slowed by poor system performance.