Exchange 2016 on-premise mailbox access using Graph API (Hybrid Setup)

Question

I have a mailbox in on prem exchange server (which is in hybrid mode) abc@onprem.com and i am trying to access this via graph api (/messages). This works perfec

Answer 1

The issue is actually somewhere else - Exchange doesn't seem to support client_credentials flow. You can, however force it via following PowerShell (make sure to restart your IIS after applying):

$apps = Get-PartnerApplication
# Microsoft Graph is 2nd item in the array, if you are unsure, list the items by calling $apps first
$apps[1] | Set-PartnerApplication -AppOnlyPermissions $apps[1].ActAsPermissions

The full explanation can be found here: https://blog.thenetw.org/2019/05/13/using-client_credentials-with-microsoft-graph-in-hybrid-exchange-setup/