MailboxNotEnabledForRESTAPI when using Graph Calendar API on on-premise Exchange

The Microsoft Graph Calendar API is not supported for on-premises or hybrid Exchange mailboxes. If you encounter the error MailboxNotEnabledForRESTAPI, it indicates that the mailbox is either inactive, soft-deleted, or hosted on-premises, which means it cannot be accessed via the Graph API.

For accessing on-premises calendars, the recommended approach is to use Exchange Web Services (EWS). EWS provides the necessary functionality to interact with mailboxes hosted on Exchange servers, including those in hybrid deployments.

Currently, there are no configurations that would allow the Microsoft Graph API to work with on-premises mailboxes in a hybrid setup. Microsoft has guidelines that specify these limitations, and it is advisable to consult those for detailed information on supported scenarios.

To correctly advise your customers, you should inform them that migrating to a fully cloud-based Exchange Online mailbox would be necessary to utilize the Microsoft Graph Calendar API. Otherwise, EWS remains the viable option for accessing on-premises mailbox data.

References:

• REST API is not yet supported for this mailbox error for request to a mailbox
• Microsoft 365 Copilot and on-premises mailboxes

Hi Alex,

Thank you for sharing your question. I understand how frustrating it can be to encounter the MailboxNotEnabledForRESTAPI error when integrating calendars, especially in hybrid or on-premises environments. This error occurs because Microsoft Graph Calendar API is designed to work only with mailboxes hosted in Exchange Online. On-premises or hybrid mailboxes do not support Graph REST APIs for calendar access. I am here to help.

The reason behind this limitation is that Graph relies on cloud services and modern authentication provided by Microsoft 365. On-premises Exchange servers do not expose the necessary endpoints for Graph, and as of March 2023, Microsoft officially deprecated REST API support for hybrid deployments. This means there is no configuration that enables Graph Calendar API to work

For accessing calendars in on-premises Exchange today, the recommended approach is to use Exchange Web Services (EWS). EWS remains supported for on-premises Exchange Server and is the best option for calendar integration until its planned deprecation in Exchange Online by October 2026. If your customers are fully on-premises, EWS with OAuth or NTLM authentication is the standard solution. In hybrid environments, you can consider migrating mailboxes that require Graph integration to Exchange Online, as Graph is the preferred API for cloud mailboxes.

I hope this gives you an idea.

Best Regards,

Noel