M365 Changelog: (Updated) REST API for On-Premises Mailboxes Preview Ending

MC338988 – Updated March 14, 2023: Microsoft has updated the timing of this change. Thank you for your patience.

Upcoming, Microsoft will be retiring the preview program enabling REST API access to on-premises mailboxes for Hybrid Exchange organizations.

Note: This will impact those who write code against Exchange mailboxes.

Key points

  • Timing: Beginning early June (previously early March), Microsoft will begin to return errors for any requests made for mailboxes that remain on-premises.
  • Action: Use Microsoft Graph for Exchange Online and Exchange Web Services (EWS) for Exchange Server on-premises
  • Roll-out: tenant level

 How this will affect your organization:


What API(s) should I use to work with Hybrid Orgs, where some mailboxes are in Exchange Online, and some are on Exchange Server?

  • You should continue to use Microsoft Graph to work with Exchange Online mailboxes and use EWS to work with Exchange Server (on-premises) mailboxes.

Wasn’t EWS going away?

  • In 2018 Microsoft announced EWS in Exchange Online would no longer receive feature updates. But the company has no plans currently to change the way EWS works on-premises, so using Microsoft Graph for Exchange Online and EWS for Exchange Server on-premises is the current recommendation.
  • If you choose to continue using EWS against Exchange Online mailboxes, you should ensure that it uses OAuth (as Basic Authentication is being turned off in Exchange Online). You should also know that it now allows restricting access to mailboxes access through mailbox access policies.

Where can I get help on using Microsoft Graph?

Learn more:

Additional information