Hello,
According to your operation, in order to prevent the assistant to view the email information, you could set the DelegateSentItemsStyle to 1 and set the MessageCopyForSentAsEnabled value to False, so that the email is only saved in the shared mailbox, and nothing is saved in the primary mailbox.
For more information, please refer to https://learn.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/sent-mail-is-not-saved
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