# Error: MailboxNotEnabledForRESTAPI (Microsoft 365)

Source: https://catalystgroup.gg/docs/email-delivery/error-mailbox-not-enabled-for-rest-api/
Updated: 2026-10-19 · By Duane Garner

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.

 

## Related

 
- [Microsoft 365 / Outlook](https://catalystgroup.gg/docs/email-delivery/mailer-microsoft-365/)
- [Error: ErrorSendAsDenied (Microsoft 365)](https://catalystgroup.gg/docs/email-delivery/error-send-as-denied/)
- [Sender name, address and the Force settings](https://catalystgroup.gg/docs/email-delivery/sender-name-and-address/)
- [Troubleshooting email delivery](https://catalystgroup.gg/docs/email-delivery/troubleshooting/)