Are we to assume that you could send email before and that this problem has just happened,or have you never been able to send email with Outlook 2003?
Again presume you can receive email in Outlook.
Send problems are often a problem with the smtp details in the account settings. Make sure that you check these and they are correct for your ISP. Note your ISP may be different from your email provider, but you use the ISP smtp details to send email and the email providers to receive.