The following online documentation provides steps on how to register GPOADmin for Office 365 Exchange Online.
https://support.quest.com/technical-documents/gpoadmin/5.21/user-guide/65#TOPIC-2312671
Step # 7 from the above online documentation states that Mail.ReadWrite and Mail.Send permissions are required, but if you need to limit this, please see Microsoft Documentation listed below for details on limiting permissions to specific Exchange Online mailboxes.
Microsoft Documentation:
https://learn.microsoft.com/en-us/graph/auth-limit-mailbox-access
Note:
The Enforce approver account validation option found when configuring email notifications will not function if you select to follow the Microsoft article to restrict access to a single mailbox.
