Thanks for the information @dneuhaeuser , we will test this soon.
Question: What about deleted items in shared mailboxes? Will it be possible to handle those in the same way?
So far, we have been managing this by applying the following changes for each user (via GPO, manually once during setup, or at login):
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Preferences" /v DelegateSentItemsStyle /t REG_DWORD /d 1 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General" /v DelegateWastebasketStyle /t REG_DWORD /d 4 /f
And what about drafts in shared mailboxes?