In order to determine the user who sent a shared mailbox message, we suggest adding a property of sent messages indicating the user who sent it as a delegate; e.g., message.delegateSender or message.actualSender.
Our app reports on shared mailbox activity, including who is sending replies and new messages. Currently, since Graph does not give us access to shared mailbox message sender information, we are forced to do it via the addition of internet message headers with an Outlook add-in and (since the Sent Items message objects don't include the header we added) an Exchange rule that Bcc's outgoing mailbox messages back to the mailbox.