I'm using Outlook 2007, but it might be similar to you:
Open a new mail message, right click on text field --> Paragraph.. the window below will open:
Default button at the bottom of the window, you should receive the message below:
Yes to change it to NORMAL mode.
EDIT: Searching for more info on Outlook 2003 I found some help on MS support here, but only when using Word as mail editor (which is not your case).
I also found this information, but I can't guarantee this will work (because I have only MSO2007 version, so I can't test it).
Please, test it and let me know if this is where you can set the NORMAL mode/style.