A family of Microsoft word processing software products for creating web, email, and print documents.
You need to modify the Envelope Return style, which is used for the return address. Press Ctrl+Shift+S to open the Apply Styles dialog. Type "Envelope Return" and then click Modify. In the Modify Style dialog, change the font as desired, select the radio button for "New documents based on this template" (assuming you want this as the default for all envelopes), and click OK.