When printing an email from Outlook 2003, you cannot change default printer. There is a workaround to this, but as Microsoft have acknowledged that it’s as bug there is no fix.
Specifically, it only happens when you print from the message preview pane in Outlook 2003 and the email contains HTML formatting.
To work around this you can do one of the following:
Open the message in full to print it, rather than print from the message preview pane
Print from the file menu instead, changing the printer from the Table Style menu and selecting memo Style from the Print Style menu – this is Microsoft’s workaround if you cannot change the printer in Outlook 2003. Personally I find option 1 easier and faster
Change the default printer, prior to printing
There are probably other ways to work around the fact you cannot change the printer from the Outlook 2003 print menu.