Much the same thing happens to me. I use Outlook in Office 365. With that I have 5 e-mail accounts set up. The default seems to make no difference and new e-mails seem to go from whichever Inbox I have chosen at the time. I have got used to simply selecting the one I need from the drop down list, if necessary. Maybe others will provide a solution for both of us.
Samsung Galaxy A9 review: Hands-on