iPhone email fonts and Desktop fonts are different

iPhone email fonts and Desktop fonts are different

Of the less than ~10 fonts for both iPhone / desktop, there is not one overlapping font.  Thus, if I reply on my iPhone to a customer, the body of the email will always look different than the signature wording?  How can I fix this asap?

iPhone 10.0.2 / with the latest version of Zoho Mail.

Thanks,

Jeff