EXCHANGE ONLINE · TECHNOLOGY ALERT
Exchange Online EWS Changes: Is Your Cross-Tenant Calendar Sharing Ready?
Microsoft begins phased Exchange Web Services disablement from October 1, 2026. Organizations using cross-tenant Free/Busy, MailTips or Calendar Sharing should check whether their existing configuration still depends on EWS.
IMPORTANT TIMELINE
EWS disablement starts October 1, 2026
October 1 is the beginning of Microsoft's phased EWS disablement—not the final shutdown. Microsoft currently plans to fully disable EWS in Exchange Online in April 2027.
EXECUTIVE SUMMARY
Why organizations should check this now
Cross-tenant calendar availability can be one of those Microsoft 365 services that quietly works for years without administrators needing to revisit the underlying configuration.
That makes the EWS retirement easy to overlook.
Microsoft is changing the mechanism used for certain cross-tenant Free/Busy, MailTips and Calendar Sharing scenarios. The replacement is Microsoft 365 Cross-Tenant Access Policy, moving these interactions away from legacy EWS and toward a modern Entra-governed model.
If your organization maintains sharing relationships with subsidiaries, partners, vendors or acquired businesses using separate Microsoft 365 tenants, this is worth reviewing before EWS disablement reaches your environment.
WHAT IS CHANGING?
Cross-tenant sharing is moving away from EWS
Exchange Web Services has been on Microsoft's retirement path for several years. As EWS is removed from Exchange Online, Microsoft must also replace Microsoft 365 capabilities that still use EWS behind the scenes.
One affected area is cross-tenant collaboration between Microsoft 365 organizations.
Free/Busy
Users checking whether people in another Microsoft 365 tenant are available for a meeting.
Calendar Sharing
Calendar information intentionally shared between separate Microsoft 365 organizations.
MailTips
Relevant Exchange information displayed while users communicate across organizational boundaries.
WHO SHOULD CHECK?
This does not affect every Microsoft 365 tenant
The specific cross-tenant change is relevant when an organization shares Free/Busy, calendars or MailTips with another Microsoft 365 organization.
Examples include organizations that:
- operate multiple Microsoft 365 tenants;
- share calendar availability with a partner organization;
- maintain separate tenants after a merger or acquisition;
- have subsidiaries operating independent Microsoft 365 environments;
- use Organization Relationships for cross-tenant Free/Busy or MailTips;
- use Sharing Policies for external calendar-sharing scenarios.
Normal calendar sharing inside a single Microsoft 365 organization is not the scenario addressed by this particular migration.
BUSINESS IMPACT
What happens if an affected configuration is left unchanged?
Microsoft says that as EWS is gradually disabled, cross-tenant Free/Busy, MailTips and Calendar Sharing that still depends on EWS can stop working.
Users may experience the issue as an ordinary Outlook or scheduling problem rather than recognizing it as an infrastructure change.
Users may no longer see partner-tenant availability while scheduling meetings.
Existing cross-organization calendar-sharing experiences may stop working.
Help desks may initially investigate Outlook while the underlying problem is the retired EWS dependency.
TIMELINE
Key dates to understand
Microsoft rolls out the new Cross-Tenant Access Policy capability. Worldwide Free/Busy rollout was expected to complete September 23.
Microsoft begins gradually disabling EWS in Exchange Online.
Hard deadline for completing the transition before final Exchange Online EWS shutdown.
ACTION PLAN
What should Microsoft 365 administrators do?
Avoid making configuration changes simply because EWS is being retired. First determine whether the organization is actually using an affected cross-tenant sharing configuration.
Identify existing sharing
Review Organization Relationships, Sharing Policies and other cross-tenant Exchange sharing configurations.
Identify partner tenants
Record the organizations, domains and business relationships that depend on cross-tenant sharing.
Determine required access
Understand what users currently need to see across tenant boundaries rather than recreating unnecessary access.
Build the replacement
Configure the appropriate Microsoft 365 Cross-Tenant Access Policies for the required collaboration scenarios.
Work with partner organizations
Where sharing is bidirectional, coordinate configuration and testing with administrators on both sides.
Test before retiring legacy configuration
Validate Free/Busy, Calendar Sharing and MailTips as applicable before removing old configurations.
NOT READY BY OCTOBER?
Microsoft provides a temporary transition path
Microsoft recognizes that organizations with many partner tenants may not be able to complete migration before October 1.
Microsoft's current guidance provides a temporary way to keep affected EWS-based cross-tenant sharing operating while migration is completed.
Microsoft currently identifies April 1, 2027 as the hard deadline for completing the move away from EWS.
WHY THIS MATTERS
Don't wait for users to discover the dependency
Platform retirements frequently expose dependencies that organizations did not realize were still in use.
Cross-tenant calendar sharing is a good example. Users may depend on it every day while the underlying Exchange configuration remains untouched for years.
A focused review now can establish whether your organization is affected, which external relationships depend on EWS and how much migration work is actually required.
HOW NOAVAXIS CAN HELP
Need to know whether your Exchange environment is affected?
NOAVAXIS TECHNOLOGY can review existing Microsoft 365 and Exchange Online sharing configurations, identify EWS-dependent relationships and provide a clear migration path.
- Review existing Exchange Online sharing configurations
- Identify cross-tenant dependencies
- Document partner-tenant relationships
- Plan Cross-Tenant Access Policy migration
- Coordinate and test replacement configurations
- Provide ongoing Microsoft 365 operational support
Find → Fix → Manage → Automate
Talk to an ExpertOFFICIAL SOURCES
Microsoft guidance
This advisory is based on Microsoft's Exchange Online EWS retirement guidance and Exchange Team guidance covering the move of cross-tenant Free/Busy, MailTips and Calendar Sharing to Microsoft 365 Cross-Tenant Access Policy.
Last reviewed: September 25, 2026. Microsoft cloud service timelines can change. Administrators should confirm current Microsoft documentation before making production configuration changes.