Question about Outlook Express

I use OSX 10.8.5 and have recently installed MS Office 2011.  I use Outlook for business and have three separate email accounts.  Whenever I REPLY to an email, Outlook arbitrarily will assign a From mailbox account, even after I have used the drop down box to select the appropriate From account.  For instance when I'm doing a reply message to Businessaccount#1, It will say on the From line Businessaccount#1 on behalf of Businessaccount#2.  It is very important that my messages are sent from the appropriate business account.  This only happens when I REPLY to emails - not creating emails.