Help us shape Exchange Server on-premises to different online org Free/Busy after EWS retirement

As mentioned in our article about Exchange Online EWS, titled “Your Time is Almost Up,” it’s important to note that Exchange Web Services (EWS) in Exchange Online is set to undergo phased deactivation starting in October 2026, with complete disablement expected by April 2027. During this transition, we’re focusing on the remaining collaboration scenarios in Exchange Server that still rely on the Exchange Online EWS endpoint.

The good news is that most Free/Busy scenarios between Exchange Server will remain unaffected. If you’re using two on-premises Exchange Server organisations, they’ll continue sharing Free/Busy information using Federation, DAuth, and Organisation Relationships. For those with a hybrid Exchange set-up, Free/Busy sharing between an Exchange Server and its connected Exchange Online tenant will still be possible. This will utilise OAuth and IntraOrganizationConnectors through the dedicated Exchange hybrid app, with Microsoft Graph taking the place of EWS in modern hybrid workflows.

However, one scenario that needs a new solution involves an Exchange Server organisation trying to retrieve Free/Busy or MailTips from another organisation’s Exchange Online tenant using an EWS-related Organisation Relationship. To tackle this external Exchange Online situation, we are considering the use of Microsoft 365 Cross-Tenant Access Policy. This policy is becoming the standard for sharing Free/Busy information, calendar events, and MailTips among Microsoft 365 tenants.

Before we make any final decisions, we want to hear from you — especially if your organisation runs Exchange Server and doesn’t already have an M365 tenant.

We encourage you to provide your feedback on this approach by filling out this form: Survey.

Note: This feedback request focuses specifically on collaboration between Exchange Server (Organisation A) and a different Exchange Online tenant (Organisation B). There will be no changes to Exchange Server-to-Exchange Server Organisation Relationships (involving two purely on-premises organisations), which will continue as they are.

The Exchange Team