Hi @davrion ,
Please try assign full access permission to a single mailbox with:
Add-MailboxPermission -Identity "ServiceAccount" -User "User01" -AccessRights FullAccess -InheritanceType All
If that doesn't work, please try to reassign the permission or create a new test user and assign full access permission to your account and test if this time it works.
If that works, you can run this command to assign permission to all users.
Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox') -and (Alias -ne 'ServiceAccount')} | Add-MailboxPermission -User ServiceAccount@domain.com -AccessRights fullaccess -InheritanceType all -AutoMapping:$false
Regards,
Lou
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