To implement font fallbacks, you need to specify a list of fonts in the CSS of your email template. The browser or email client will attempt to load the fonts in the order specified. If the first font fails, it will try the next one, and so on. Here's a simple example:
body { font-family: 'Open Sans', Arial, sans-serif; }
In this example, 'Open Sans' is the preferred font. If it fails to load, Arial will be used. If Arial is also unavailable, the generic sans-serif font will be used.