This is not a problem, this is a “feature”. There is no way to make this work from within your profile. You have to set up a profile as the other mailbox and open outlook with that second profile in order for sent items to stay in that folder and say they came from that mailbox. Close outlook, go int the mailbox icon in the control panel, click “show profiles” and select “prompt for profile” instead of “always use this profile”. Then open Outlook and when the prompt window comes up choose “new” and create the other profile. Then whenever you want to send from that mailbox close outlook and re-open it on that profile.