You wish to utilise Folder Permissions Manager in a Hybrid Exchange on-premise / Exchange online environment but would like to understand the limitations and requirements of this configuration.
As a first step in diagnosis, please ensure you are using the latest version of Folder Permissions Manager. We will be unable to provide troubleshooting for older versions.
Scenario
You wish to utilise Folder Permissions Manager to apply permissions from a cloud user/distribution list/security group to an on-prem mailbox, or vice-versa, but cannot apply permissions other than Full Access or Send on Behalf to the mailbox.
Cause
As detailed in the following Microsoft article, it is not possible to assign cross-domain (cloud to on-prem or on-prem to cloud) permissions using Outlook or the EWS console, even if the accounts in question are synced across domains.
This information is covered in more detail in the following Microsoft article concerning cross-domain permissions.
Permissions in Exchange hybrid environments
Attempting to utilize cross-domain permissions can result in the following errors in File Permissions Manager:
User or Group is not found when searching for user
Email Address is not populated for a group
This permission could not be applied because the user identifier is invalid
Scenario
You are attempting to use Folder Permissions Manager to apply permissions to a user located in Entra ID, however, the user is not displayed when using the search function
Cause
Users must be present within the on-premises AD domain to be visible within FPM - users that only exist within Entra ID cannot be listed by the search function as this is unable to connect to Entra ID.