To specify fallback fonts, you can use the font-family CSS property in your email's HTML code. For example:
p { font-family: 'PrimaryFont', 'FallbackFont', 'AnotherFallbackFont', sans-serif; }
In this example, the email will first attempt to use 'PrimaryFont'. If it cannot load, it will fall back to 'FallbackFont', and so on. The final option, generic fonts like 'serif' or 'sans-serif', ensures that a readable font is always used.