Shared Microsoft 365 calendars and availability between separate organisations could stop working from October 2026 if the connection still relies on Exchange Web Services (EWS).
Microsoft materially updated Message Center notice MC1446796 on 25 August 2026. It says cross-tenant Free/Busy information, calendar sharing and MailTips are moving to a newer Microsoft 365 Cross-Tenant Access Policy.
This does not affect every business. It matters where one Microsoft 365 organisation deliberately shares calendar or availability information with another tenant—for example, related companies, a client and supplier, or organisations with a long-standing sharing arrangement.
What is Microsoft changing?
Microsoft is retiring EWS in Exchange Online because it is an older, highly privileged way for applications and services to reach mailbox data. The wider EWS retirement begins on 1 October 2026.
For cross-organisation calendar sharing, Microsoft is replacing the EWS-based connection with the Microsoft 365 Cross-Tenant Access Policy. This uses Microsoft Graph and more modern authentication.
The important point for businesses is simple: an existing sharing arrangement may have worked quietly for years, but it will not automatically keep working just because both organisations still use Microsoft 365.
Which Microsoft 365 calendar features could be affected?
Microsoft says the migration covers several familiar Exchange Online sharing features:
- Free/Busy information — whether somebody in another organisation appears available, busy or out of office.
- Calendar sharing — shared calendar details made available between separate Microsoft 365 tenants.
- MailTips — warnings and helpful information shown before an email is sent, including some out-of-office information.
Microsoft’s migration report can also identify anonymous calendar publishing and other legacy sharing settings that need separate review. These should not be assumed to migrate in exactly the same way.
When could shared calendars stop working?
Microsoft’s current timetable is:
- August to 15 September 2026: worldwide rollout of the replacement cross-tenant sharing capability.
- From 1 October 2026: Microsoft begins gradually disabling EWS in Exchange Online.
- 1 April 2027: final EWS shutdown, including for organisations that requested the temporary extension.
Microsoft allows administrators to request a temporary EWS extension by setting EWSEnabled to true, but this is a contingency rather than a permanent solution. Businesses should use the extra time to migrate and test, not treat it as a long-term fix.
Microsoft 365 Review
Do your shared calendars depend on EWS?
KES can check your tenant relationships, legacy calendar-sharing settings and migration route before users lose visibility of external availability.
- Tenant configuration check
- Calendar continuity planning
- Safe cross-tenant migration
- Clear communication for users
Not every tenant is affected
This specifically concerns calendar, availability or MailTips sharing between separate Microsoft 365 organisations.
Read our wider EWS retirement guide →What might users notice if nothing changes?
The effect can look like an ordinary Outlook or calendar fault. Users may find that:
- people in a partner organisation show as unavailable or their status cannot be retrieved;
- a calendar shared from another Microsoft 365 organisation no longer opens or updates;
- out-of-office information and other MailTips from an external organisation stop appearing;
- meeting planning takes longer because staff cannot see when external colleagues are free.
These symptoms can be confusing because internal calendars in the same tenant may continue to work normally. The problem is the connection between the two organisations.
Does this affect all Microsoft 365 businesses?
No. Ordinary calendar sharing between users in the same Microsoft 365 tenant is not the focus of this change.
The notice applies to cloud-to-cloud Exchange Online arrangements that use legacy organisation relationships or sharing policies to exchange information between separate tenants. Microsoft says hybrid and on-premises Exchange environments follow different guidance and are not part of this particular migration route.
If your business has never deliberately configured cross-organisation calendar sharing, there may be nothing to migrate. An administrator should confirm this rather than relying on guesswork.
What should businesses do before October 2026?
- Ask your IT provider to inspect organisation relationships and sharing policies. Microsoft provides a migration assessment report that can identify affected settings.
- List the organisations you share availability or calendars with. Include group companies and long-standing partner relationships that staff may take for granted.
- Coordinate with the other tenant. Bidirectional sharing requires complementary configuration on both sides.
- Migrate and test before EWS is disabled. Check Free/Busy lookups, shared calendar detail and MailTips with real test users.
- Use the extension only as a safety net. It ends on 1 April 2027 and does not remove the need to migrate.
- Tell users what to expect. A short explanation can prevent calendar-sharing failures being mistaken for random Outlook problems.
For a broader view of the retirement, see our guide to Microsoft switching off EWS. Businesses that want ongoing help can also review our Microsoft 365 management service.
Quick answers
Will normal internal calendars stop working?
No. This notice is about sharing between separate Microsoft 365 organisations, not ordinary calendar use inside one tenant.
Why is Microsoft making the change?
EWS is a legacy, highly privileged interface. Microsoft is moving cross-tenant calendar sharing to Microsoft Graph and a newer cross-tenant policy.
Can a business delay the change?
A temporary EWS extension is available, but it ends on 1 April 2027. It should be used only to create time for a controlled migration.
Does each organisation need to take action?
Where availability or calendars are shared in both directions, both tenants normally need complementary policies. Coordination with the other organisation is therefore essential.
The important point
This is not a reason for every Microsoft 365 user to panic. It is a deadline for organisations that deliberately share calendar information, Free/Busy status or MailTips across tenant boundaries.
Because the existing feature is quiet and familiar, businesses may not realise it depends on EWS until users report that external calendars or availability have disappeared. Checking the configuration before 1 October is far easier than diagnosing the failure afterwards.
Microsoft sources
- Microsoft 365 Message Center notice MC1446796: Migrate Free/Busy, MailTips, and Calendar Sharing before EWS deprecation (public archive; materially updated 25 August 2026).
- Microsoft Learn: Migrate from Exchange Online Exchange Web Services to Microsoft 365 cross-tenant access policies.
- Exchange Team Blog: Exchange Online EWS — your time is almost up.
Microsoft can refine rollout dates and technical guidance. KES will update its advice if the published timetable changes.
