How to change the default font in Microsoft Outlook from 'Calibri' to one you prefer more

To change the font in Outlook, head to the "Signatures and Stationery" menu, where you can pick from a number of fonts.
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Thunderbird won't import from Outlook b/c Outlook is not default. How to?

How do I set Outlook (2000) to the default mail client?<BR><BR>Thunderbird can't import mail from Outlook unless Outlook is the default, but I can't find where to force that.