I've been through every option in my Outlook 2003 and could not find anything that would change the way replies are handled, in reference to the e-mail account that is used.  One final suggestion.  Delete your primary (default) e-mail account and then create a new account, set it as default and see if that helps.  About the only other remote possibility.   
When I use Outlook to reply to e-mail, I understand that the reply is supposed to be sent from the same account that received the original mail. That doesn’t happen. When I reply to a message that was sent to my default e-mail account, the reply is sent from another account. In order to have it sent from my default account I must manually click Accounts and select the default account before clicking Send. How can I make Outlook 2003 (under Vista Home Premium) automatically use the default account when replying to a message sent to that account?
When replying to an e-mail message, Outlook sends from the wrong account