Why does Outlook 2003 always reply with my default account?

I’ve got two accounts configured in Outlook 2003. First is my company account on our Exchange server and it’s setup to be the default. The second is a personal POP3 account. Problwm is, whenever I reply to a message, it replies from my Exchange server account, even if the message was sent to the POP3 account. I’ve read that Outlook is supposed to reply to the account the message was sent to, but that’s not the case here. Anybody have any suggestions?