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What a year we have had! Who would have thought at the beginning of 2025 that Exchange administrators would be sweating so much? Even though much of it could have been avoided.
Centralized Mail Flow (also known as Centralized Mail Transport, CMT) is an option in Exchange hybrid environments whereby all outgoing Internet messages from mailboxes in Exchange Online are first routed through the local Exchange organization before being delivered to the Internet. Similarly, depending on the MX strategy, incoming Internet messages can first pass through the local environment before being delivered to cloud mailboxes. The goal is usually to continue applying central compliance, DLP, encryption, journaling, or gateway functions in the local infrastructure. As a rule, CMT is configured as part of the hybrid configuration using the Hybrid Configuration Wizard (HCW).
Microsoft is changing how hybrid authentication works between on-premises Exchange servers and Exchange Online. The long-standing Shared Service Principal (“Office 365 Exchange Online”, App ID 00000002-0000-0ff1-ce00-000000000000) will no longer be supported. Each tenant must deploy a dedicated Entra enterprise application by October 31, 2025.