Microsoft Graph API returns ErrorInvalidUser for valid mailbox user while FindMeetingTimes succeeds

We are using Microsoft Graph API with Application Permissions (Client Credentials Flow).

The access token is successfully generated and contains the following:

• Audience: https://graph.microsoft.com/
• Permission: Calendars.ReadWrite (Application)
• Token type: Application Token

We are able to successfully call the FindMeetingTimes API and retrieve available meeting slots for users.

Example:

POST /v1.0/users/{organizer}/findMeetingTimes

Response:

• Returns available meeting slots successfully.

However, when attempting to create or access calendar events for a user mailbox, Microsoft Graph returns the following error:

{
"error": {
"code": "ErrorInvalidUser",
"message": "The requested user '[gaurav.anand@sinch.com](mailto:gaurav.anand@sinch.com)' is invalid."
}
}

Endpoints affected:

GET /v1.0/users/gaurav.[anand@sinch.com](mailto:anand@sinch.com)/calendar/events

and

POST /v1.0/users/gaurav.[anand@sinch.com](mailto:anand@sinch.com)/events

Observations:

1. The user mailbox exists and the UPN is confirmed to be correct.
2. The application token is successfully generated.
3. FindMeetingTimes API works successfully using the same application and token.
4. Calendar event APIs fail with ErrorInvalidUser for the same mailbox.
5. We would like assistance understanding why Graph can resolve availability information but returns ErrorInvalidUser when accessing the mailbox calendar directly.

Please help determine whether this is related to mailbox resolution, Exchange Online configuration, Application Access Policies, tenant configuration, or a Microsoft Graph issue.

No repository file, test, or entry point is identified; the report concerns Microsoft Graph behavior outside the Graph Explorer codebase. Start by reproducing the listed GET and POST calendar endpoints with the same application token and checking the referenced mailbox and tenant configuration. Done means identifying whether the failure is caused by mailbox resolution, application access policy, tenant configuration, or Graph itself.