When you specify fonts in your email's CSS, you can list multiple fonts in a font-family property. The email client will try to use the first font in the list. If that font is not available, it will move to the next one, and so on, until it finds a font that is available on the user's device. For example:
font-family: 'CustomFont', Arial, sans-serif;
In this example, if 'CustomFont' is not available, the email client will use Arial. If Arial is also not available, it will fall back to the generic sans-serif font.