Fix "MailboxNotEnabledForRESTAPI" in Power Automate and Dataverse

A flow step from the Office 365 Outlook connector fails with error code MailboxNotEnabledForRESTAPI and the message "REST API is not yet supported for this mailbox." Or a mailbox in Dataverse fails its test with error 8005E274. Your flow's logic is fine. The connector works with mail through Exchange Online's REST API (and Microsoft Graph), and the mailbox it reached isn't one that API serves, usually because it lives on an on-premises Exchange server or the account has no Exchange Online licence. The fix is on the mailbox or the account, which normally means an Exchange or Microsoft 365 admin.

• An Office 365 Outlook action, such as sending an email or getting emails, fails with the message REST API is not yet supported for this mailbox. Microsoft documents it as an HTTP 404.
• The run shows the code MailboxNotEnabledForRESTAPI or MailboxNotSupportedForRESTAPI.
• It fails for one connection or one mailbox, while the same flow works with another.
• In Dataverse, testing and enabling a mailbox fails with error code 8005E274.

What the error means

Microsoft documents the message for any request to a mailbox through the Outlook REST API: HTTP error 404, one of the two codes above, and "REST API is not yet supported for this mailbox." Its stated cause is a mailbox that's on a dedicated Microsoft Exchange Server and isn't a valid Microsoft 365 mailbox.

The Office 365 Outlook connector depends on that API. Microsoft's connector page says that to use it you need access to an Office 365 mailbox with the REST API enabled, and some of its operations go through Microsoft Graph. So the error isn't about the action's settings, the email's content or your permissions on a mailbox. It's about where the mailbox lives and whether Exchange Online can serve it.

Find the mailbox behind the connection

The action reaches a mailbox through its connection, so start there:

1. Open the flow's run history, select the failed run, and select the failed Outlook action.
2. Note the connection it uses, and the account that connection signs in with. That account's own mailbox is the one the action reaches, unless the action names another.
3. If the action has a shared mailbox address filled in, that shared mailbox is checked too. Note it.
4. Ask a Microsoft 365 admin to look up each account in the Microsoft 365 admin center: whether it has a licence that includes Exchange Online, and whether its mailbox is in Exchange Online or on an on-premises server.

A useful test: create a connection with your own licensed work account and run the same action. If it works, the flow is fine and the original account or mailbox is the problem.

Causes and fixes

Microsoft's pages give these causes. Each is fixed outside the flow:

One version to watch for is a service account created just for flows and licensed for Power Automate but not for Exchange Online. It can sign in, so the connection looks healthy, but it has no Exchange Online mailbox for the action to reach.

Shared mailboxes

A shared mailbox in Exchange Online doesn't need its own licence (up to 50 GB), but Microsoft says a person must have a licensed Exchange Online mailbox to access one, and that a shared mailbox's own account isn't meant for signing in. So:

• Don't sign the connection in as the shared mailbox's account. Sign in as a licensed user, or a licensed service account, that has permission on the shared mailbox.
• Put the shared mailbox's address in the action, for example Send an email from a shared mailbox (V2).

For the permissions themselves, and the "send on behalf" and "not found in the store" errors, see send email from a shared mailbox in Power Automate.

Related connector errors

Microsoft's Office 365 Outlook connector page lists two errors that look similar but have different fixes:

In Dataverse server-side synchronization

The same code appears when Dataverse syncs email with Exchange. When you test and enable a mailbox, the test fails with error code 8005E274, and the alerts on the mailbox record can show Email Server Error Code: MailboxNotEnabledForRESTAPI. Microsoft gives the same causes (no Exchange Online licence, a disabled mailbox, or a mailbox in an Exchange on-premises hybrid deployment) and this fix:

1. Check that the person has an Exchange Online licence. If you assign one, allow time for it to sync and for Exchange to create the mailbox.
2. For a mailbox that stays on-premises in a hybrid deployment, create a server profile that points to the on-premises Exchange server. In the Power Platform admin center, open the environment's Settings, then under Email select Server profiles, and New server profile. Set the email server type to Exchange Server (on-premises), give the profile a name, and save it.
3. Test and enable the mailbox again, and if it still fails, read its alerts.

For other failed runs, our cloud flow error codes guide covers the common status codes.

Sources

• REST API is not yet supported for this mailbox error for request to a mailbox — Microsoft Learn
• Troubleshoot common issues with email in flows — Microsoft Learn
• Office 365 Outlook connector: common errors and known issues — Microsoft Learn
• Error code 8005E274 when you test and enable mailbox — Microsoft Learn
• About shared mailboxes: licensing and sign-in — Microsoft Learn