Fonts need to exist on the device that receives / reads the email, or the email client needs to insert a web accessible reference to the desired font. It can do this via import statements or other syntax:
@import url`('https://fonts.googleapis.com/css?family=Open+Sans');
@import url`('https://fonts.googleapis.com/css?family=Calibri');
<link href="https://fonts.googleapis.com/css?family=Open+Sans" rel="stylesheet">
... more
If the receiver doesn’t have the font on the device and no external reference has been included in the email, the receiver will fallback to some other (default) font. Frequently Times New Roman, but it depends on the device and the app.
From looking at the source of a message sent by EMC, it does not insert an external (web accessible) reference. Some email clients can do this; others not.
Also, even with a web accessible font specified in the email, the client needs to support that mechanism for obtaining the font. Again, depends on the client.
p.s. You might be able to use a mail template to help you out. Not sure. I’m just taking a look at them.