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Please try opening NormalEmail.dotm and setting the Font Theme to Segoe UI Semilight using Design>Fonts>Customize Fonts. Then set Normal style to Segoe UI Semilight (Body). The font dropdown for all styles should read Segoe UI Semilight (Body) or Segoe UI Semilight (Headings).
Setting NormalEmail.dotm only handles new emails. When replying to emails, the font theme has been set by the sender's template, if they are using Outlook. So you'll still get Calibri when replying to many messages.