MS Graph API with Resource Calendar

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MS Graph API with Resource Calendar

hello.

I am building a service using PHP to receive calendar webhooks when events are registered or modified in Outlook via the MS365 Graph API.

Currently, I am getting an error that some of my customers who subscribe to the calendar through the service are unable to receive events in their mailboxes.

When I delete the subscription and request again, there is no error in the response.

On user lookup, the email address resolves fine, but on calendar/event lookup, it shows not found for the specific resource.

It's obviously a mailbox that's being received normally, and it's happening without any changes.

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I am building a service using PHP to receive calendar webhooks when events are registered or modified in Outlook via the MS365 Graph API.

This is supported when subscribing to user mailboxes or Microsoft 365 group calendars.Room and resource mailboxes, however, do not support subscriptions through webhooks.

Some of my customers who subscribe to the calendar through the service are unable to receive events in their mailboxes.

If the affected mailboxes are resource mailboxes (like rooms or equipment), this behavior is expected.Resource calendars cannot have subscriptions. It could also occur if the mailbox does not have an Exchange Online license.

On user lookup, the email address resolves fine, but on calendar/event lookup, it shows not found for the specific resource.

A user object can resolve even if the calendar is unavailable for subscription.A "ResourceNotFound" error can happen when trying to subscribe to a resource mailbox, when the mailbox is missing an Exchange license, or when a Graph subscription has expired (after about 71 hours).

It's obviously a mailbox that's being received normally, and it's happening without any changes.

Even if email is functioning normally, Microsoft Graph has specific support rules for subscriptions.
Room and resource mailboxes are not eligible for webhook subscriptions and should be polled manually instead.

Additionally, please verify the mailbox has an active Exchange Online license.

You can check this using Graph Explorer (web), curl from Terminal (Linux/Mac/Windows), or PowerShell (Windows).

If using curl in Terminal or PowerShell, send:

curl -X GET https://graph.microsoft.com/v1.0/users/{userId}/licenseDetails -H "Authorization: Bearer {access_token}"

If no Exchange Online service plan appears, calendar events cannot be accessed. Also ensure the subscription has not expired (subscriptions last about 71 hours and must be renewed before that). If the mailbox is a room or resource type, subscriptions are not supported. In that case, use polling instead.

To manually retrieve events, send with curl:

curl -X GET https://graph.microsoft.com/v1.0/users/{resourceId}/calendar/events -H "Authorization: Bearer {access_token}"

Notes:

{access_token} must be inserted — they must already have obtained an OAuth token via Microsoft Identity platform.

This is standard Graph access method whether Linux, Mac, or Windows.

(Information sourced from Microsoft Learn documentation: Microsoft Graph Calendar API and Notifications Support Matrix)

Damien0Reputation points
2025-04-28T05:11:11.8866667+00:00
Even though you mentioned that the absence of an Exchange Online license could cause the ErrorItemNotFound issue,
I have confirmed that the affected mailbox does have a valid Exchange Online license (EXCHANGE_S_FOUNDATION, EXCHANGE_S_ENTERPRISE) with provisioningStatus: Success.
Even though https://graph.microsoft.com/v1.0/places/microsoft.graph.room successfully lists all room resources with their emailAddress,
when I query https://graph.microsoft.com/v1.0/users(/calendar/events,
some resources (like A) return events correctly, but others (like B) return:
{ "error": { "code": "ErrorItemNotFound", "message": "The specified object was not found in the store." } }
To clarify:
Listing rooms through the places API succeeds for all rooms.
The affected mailboxes have active Exchange Online licenses.
Now, I am not trying to subscribe — I am manually polling the calendars (following your curl command).
However, querying calendar/events based on the emailAddress is inconsistent — some succeed, some fail with “ErrorItemNotFound.”
My questions are:
Why does places/microsoft.graph.room return resources that cannot be queried via users('{emailAddress}')/calendar/events?
Does being listed in the places API not guarantee calendar accessibility?
Is there any property or indicator in the places or users resource that I can check to know in advance whether a resource is actually accessible via calendar/events?

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MS Graph API with Resource Calendar

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If using curl in Terminal or PowerShell, send:

To manually retrieve events, send with curl:

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