Keeps sending from the wrong e-mail account

I have four different e-mail accounts.  Instead of sending all mail from my default account, this horrible version of outlook keeps auto sending mail from whatever e-mail address is highlighted in my inbox.  I have on numberous occasions sent out e-mails to people from an address I didn't want anyone else to know existed!  I even put in the settings  "forward and reply to mail using default e-mail account" and this very ill-written software STILL put s the wrong e-mail address in the "sending from" box.  What do I do?