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✉️ Email Delivery

Documentation, reference material and tutorials for CRM-NG SMTP Email: setup guides for every mail service, troubleshooting, and one article per error message.

Error: MailboxNotEnabledForRESTAPI (Microsoft 365)

Updated

By Duane GarnerChief Executive Officer (CEO)
Summarize: Claude ChatGPT View Markdown
On this page

The From Email mailbox is not an active Exchange Online mailbox, so Microsoft Graph cannot send from it. License it or pick another mailbox.

This article is for sites using the 365 / Outlook mailer (Microsoft Graph) where a send fails with MailboxNotEnabledForRESTAPI.

The error as you see it

MailboxNotEnabledForRESTAPI: The mailbox is either inactive, soft-deleted, or is hosted on-premise.

Shown by the 365 / Outlook mailer, in the failed-send banner, in Tools → Debug Events and in the Email Log.

What this error means

The plugin sends through Microsoft Graph as the mailbox in From Email. Graph found a user with that address, but there is no working Exchange Online mailbox behind it. Graph can only send from mailboxes that live in Microsoft's cloud and are active. Your app, secret and permissions are fine; the problem is the mailbox.

Common causes

  • The user has no licence that includes Exchange Online, or the licence was removed. Without one, the mailbox is disabled after a grace period.
  • The mailbox was just created or licensed and Exchange has not finished setting it up. This can take from a few minutes to a day.
  • The mailbox is on an on-premises Exchange server in a hybrid setup. Graph cannot send from it.
  • The user was deleted and the mailbox is soft-deleted (kept for a while in case you restore it).
  • The address belongs to a personal Microsoft account (Outlook.com, Hotmail), which this app-only mailer cannot use.

How to fix

  1. Check the user

    In the Microsoft 365 admin center at https://admin.microsoft.com/, go to Users → Active users and search for the From Email. If you only find it under Users → Deleted users, restore it or choose another address.

  2. Check the licence

    Open the user, choose Licenses and apps, and make sure a licence that includes Exchange Online is assigned. Save.

  3. Or use a shared mailbox

    For a site address such as website@yourdomain, a shared mailbox needs no licence. In the Exchange admin center (https://admin.exchange.microsoft.com/) go to Recipients → Mailboxes → Add a shared mailbox.

  4. Confirm it is a cloud mailbox

    In the Exchange admin center, Recipients → Mailboxes should list the address with a User mailbox or Shared mailbox type. If it is missing there but exists on your own Exchange server, move it to Exchange Online or pick a cloud mailbox.

  5. Wait for provisioning

    After licensing or creating a mailbox, sign in to https://outlook.office.com/ as that user (or open the shared mailbox). When the inbox loads, Graph can usually use it.

  6. Update CRM-NG SMTP

    In CRM-NG SMTP → Settings → General, set From Email to the working mailbox's primary address and press Save Settings.

  7. Send a test

    Go to CRM-NG SMTP → Tools → Email Test, fill in Send To and press Send Email.

Verify it worked

The test page shows "Test email sent successfully! Check your inbox to confirm delivery." and a copy appears in that mailbox's Sent Items.

Notes

  • If the error appears for some messages but not others, a plugin may be overriding the From address while From Email is empty. Fill in From Email and tick Force From Email.
  • If you use an Exchange Application Access Policy, add the new mailbox to the policy's group.
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