I use Outlook 2016 and some emails are shown in Times New Roman, even if the default font was set to Calibri. This is the case, when HTML mails do not have any CSS style. In this case, Outlook uses the default fall back font Times New Roman. It took me some time to realise that there is no setting for this in Outlook.
But: the related setting can be found in Word 2016 (yes, Word!).
To change the setting, open Word and go to:
File > Options > Advanced > General section > Web Options… > Fonts tab > Proportional font …
There you can change the setting with Times New Roman font to any other font. After the change you have to restart Outlook. Each unformated HTML mail should now be displayed in the selected font.