An announcement has gone out to Microsoft 365 administrators that shouldn't be overlooked. It's Message Center post MC1446796. Because it's broadcast to every tenant administrator, it's hard to tell whether it actually applies to you — but if your organization is affected and you leave it as is, calendar sharing will stop on October 1.
🤖✍️ This article was co-written with AI — an AI agent (Claude Code) organized the primary sources and drafted the piece, which Ebisuda then reviewed and edited before publishing.
Starting October 1, 2026, Exchange Online will begin phasing out EWS (Exchange Web Services). If a tenant's EWSEnabled setting is left unset (Null), it will automatically be changed to False and EWS will be blocked.
Free/busy information, mail tips, and calendar sharing with other Microsoft 365 organizations run on top of EWS, so once it's blocked, those will stop working. Furthermore, even if you haven't configured cross-organization sharing, any in-house apps or business products that use EWS will stop at the same time.
Whether cross-organization sharing applies to you can be determined with three Exchange Online PowerShell commands — Get-OrganizationRelationship, Get-AvailabilityAddressSpace, and Get-SharingPolicy. All three are read-only.
The replacement is the Microsoft 365 cross-tenant access policy, which will become available in stages starting September 2026.
If you can't make the deadline, there's a grace period: tenants that pre-configure an allow list and set EwsEnabled to True by the end of August 2026 will be excluded from the automatic change (to False) on October 1. Even if you miss that window, you can re-enable it later by setting the value back to True or Null — but April 1, 2027 is the final deadline, with no exceptions (maintaining two-way sharing also requires the partner organization to configure the same setting on their side).
I've made a video walking through how to run the diagnostic commands and how to interpret the results.
📺 [[Critical] Exchange Online EWS Retirement: How to Check Right Now Whether You're Affected [M365]](https://www.youtube.com/watch?v=SAVT7JZJRzs)
Update, August 16, 2026: Key points 2, 3, and 5 have been revised to match the primary source more precisely (clarifying that the grace period works by excluding, from the automatic change, tenants that configure an allow list and set the value to True by the end of August; and that the three diagnostic commands apply specifically to cross-organization sharing).
Please always verify the details and the latest information against the official sources.
* Exchange Online EWS: Your Time Is Almost Up (Japan Exchange/Outlook Support Team)
* Migrate to Microsoft 365 Cross-Tenant Access Policy (Microsoft Learn migration guide)
* Deprecation of Exchange Web Services in Exchange Online (Microsoft Learn)
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This article was created using "Ebi Workspace" (formerly claude-workspace), a plugin for Claude Code / Codex 👉 Ebi Workspace (keeping the memory and progress of work) / The foundation for conversation and coordination is the OSS project "Ebi Agent Chat Relay" (formerly CCDB) 👉 GitHub
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