But what the others have hinted at is that Calibri isn't one of the universal fonts. It isn't installed on every computer. Which means it isn't available in the marketo rich text editor as a standard font because it is not guaranteed to display on all browsers. So you can take a choice, 1. use an unsupported font with unpredictability. 2. choose a different font.
As Michelle started you can edit the font-family: in the email template itself. The way that works is that you specify to the computer which fonts you want shown, in order. So if you take her example, font-family: Tahoma, Helvetica, serif; you're specifying to the computer, render this in Tahoma, if you don't have Tahoma then use Helvetica, if you don't have either, then just use any serifed font.
So you'd want to change your font-family: to....
font-family: calibri, verdana, arial, sans-serif;
Then when computers get your email they'll check, do I have calibri, yes/no, if no, do I have verdana, yes/no.. etc..