Would love to see support for email markup via schema.org. Adding this gives an enhanced customer experience as Gmail displays actionable buttons inline in your inbox. In the instance of Zoho Invoice, these would ideally be Go-To actions. 

Adding a small button in the inbox next to the subject with the text 'View Invoice' which would take you directly to the invoice url without having to enter the email. 

More info here: https://developers.google.com/gmail/markup/