Exchange Online: EWS disablement affects Free/Busy from Exchange Server to another org's Exchange Online
Summary
Microsoft will disable Exchange Web Services (EWS) in Exchange Online in phases starting October 2026 and fully in April 2027. Free/Busy and MailTips from an Exchange Server organization to a different organization's Exchange Online tenant via an EWS-dependent Organization Relationship are affected. Microsoft is still evaluating a replacement path and has asked for feedback.
What changes?
Once EWS is disabled, an Exchange Server organization can no longer retrieve Free/Busy or MailTips from another organization's Exchange Online tenant through an EWS-dependent Organization Relationship. Calendar availability lookups for those external users may stop working.
Affected
- Exchange Server (Organization A) retrieving Free/Busy or MailTips from a different organization's Exchange Online tenant (Organization B) via an Organization Relationship
- Not affected: Free/Busy between two purely on-premises Exchange Server organizations (Federation/DAuth and Organization Relationships)
- Not affected: Exchange hybrid between Exchange Server and its own Exchange Online tenant (OAuth, IntraOrganizationConnectors, dedicated Exchange hybrid app, Graph replaces EWS)
Preparation and temporary exceptions
No workaround known.
Check and complete
No final solution yet. Microsoft is evaluating the Microsoft 365 Cross-Tenant Access Policy as the new path and is collecting feedback via a survey.
Recommendation
Check whether your Exchange Server organization shares Free/Busy or MailTips with other organizations' Exchange Online tenants. If so, plan for the change before April 2027, follow the Exchange Team Blog for the final direction and consider giving feedback via the survey. Hybrid setups and purely on-premises sharing need no action.
Timeline
- Phased disablement of Exchange Web Services (EWS) in Exchange Online begins.
- EWS in Exchange Online is fully disabled.
Sources
- Exchange Team Blog: Help us shape Exchange Server on-premises to different online org Free/Busy after EWS retirement Vendor · techcommunity.microsoft.com
Compiled from the sources listed and translated with AI (Claude); status and dates come from the vendor. The vendor’s statements are authoritative. Updated 1 Nov 2026, 15:57.
Where the information comes from
We continuously read the vendors’ official sources, community reports and trade media and summarise them in our own words, with a link to the source:
- Microsoft: Windows release health (Windows 10/11, Server 2016 to 2025), Windows message center, known issues of Microsoft 365 Apps (Outlook, Excel, Word, PowerPoint), Edge, Intune, Autopilot, Windows 365 and Entra Connect, Microsoft KB articles with timelines, Exchange and Windows IT Pro blogs, optionally Microsoft 365 service health and message center
- other vendors: Veeam KB, Citrix (known issues), Ubuntu and Debian (regressions), Broadcom (VMware), Fortinet, Cisco, Mozilla, Let’s Encrypt
- community (title, short excerpt and link only, without the names of the authors): Microsoft Q&A, Microsoft Tech Community (discussions), Veeam R&D Forums, Eleven Forum
- targeted web research with AI (Claude), e.g. on the updates of the month – this also finds discussions on Reddit and in forums; such entries are labelled as community reports until the vendor confirms them
- early warning from trade media (title and link only): Born’s Tech and Windows World, BleepingComputer, AskWoody, Windows Latest, Windows Central, gHacks, The Windows Club, Deskmodder, Dr. Windows, WindowsArea, Frankys Web, heise online
Summary and translation with AI (Claude by Anthropic) – without any data of visitors.
Affected? We can help.
We check whether your systems are affected, apply workarounds properly and plan for dated changes in good time.