Troubleshooting Tip: 'Error: ErrorNonExistentMailbox'

1. During integration with Exchange on-prem Via the clawback API, symptom errors occur as follows:
   'Error messages "EWS API: get scan folder; user: admin.XXX@XXX.cloud; Error: ErrorNonExistentMailbox (No mailbox with such guid.)' and 'EWS API: list people; account: fortimail_test@XX.XXX.com.XX; remote error code: 500'.

2. FortiMail is configured with HA/ A-A Setup running on a hybrid where 30% mailboxes are on-prem and 70% in the Cloud.
3. The error shows that FortiMail tried to call Exchange Web Services but got 500 (Internal Server Error).
4. Check on the throttling policy on the exchange side by using the following command:

Set-ThrottlingPolicy "EWSAPIScanPolicyHighLimit" with: EwsMaxConcurrency = 4 × {user_number} EwsMaxBurst = 10,000 × {user_number} EwsRechargeRate = 1000 × {user_number} EwsCutoffBalance = 10,000 × {user_number} Set-Mailbox "{ service_account_email_address }" -ThrottlingPolicy "EWSAPIScanPolicyHighLimit" Get-Mailbox "{ service_account_email_address }" | FL ThrottlingPolicy