The only way to have the mail sent from a delegated mailbox remain in that mailbox’s sent items is to open that mailbox as its own profile. Close Outlook, go to your mail icon in the control panel. Click “show profiles” then make the selection to have outlook prompt you for which profile to open. Open outlook again and you will get a window prompting you to choose a profile. Click “New” and create a new profile for the other mailbox. Then when you want to send from that mailbox close your outlook, re-open it and choose the other mailbox profile. The sent mail will remain in the sent items of that mailbox.