I've created a custom template (.oft) in Outlook 2007.
How can I configure it as the default template so that it will appear whenever I click the New button?
you can always put together a short macro that calls up your template
and add it as a button on your menu. it won't replace the 'new mail' button, but will be right next to it and work very similarly, just bringing up your template and no the default template.
The following article describes how to change the default messageby specifying margins, setting tabs, configuring line spacing, and more, and then applying your changes to the Normal template for Outlook messages:
However, this method doesn't use your .oft file, for which I have no direct solution.
The best I can find is this article, Create & Use Templates in Outlook 2007, which shows how to create and store templates for easy use in Outlook's Drafts folder.