Default Sending Account in Outlook 2003

We have Windows XP clients with Outlook 2003. Each client has two accounts set up: One for Microsoft Exchange Server and one for a pop3/smtp mail server external to the company. I have set up the external account as the default for sending, but Outlook still sends (sometimes) through the Exchange server.
Although I would prefer to send through the Exchange server all the time, there are various reasons why we can’t send through the Exchange Server right now and must use an external account.
Does anyone know if there is a way to prevent Outlook from ever sending mail via the Exchange account?