Hi @Ytse Jam,
From your post you are using CustomAttribute2 -eq USA as an condition of RecipientRestrictionFilter.
While when the admin creates a new recipient mailbox or group mailbox, the attribute CustomAttribute2 of the mailbox won't have the value set as USA so the admin does not have the permission to create this mailbox.
The admin, however, should still be able to manage existing mailboxes which have the CustomAttribute2 set as USA.
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