This is not a SaaS; this is an application built and distributed to more than 20 million people on their own machines, with different hardware, operating system

This is not a SaaS; this is an application built and distributed to more than 20 million people on their own machines, with different hardware, operating systems, installed software, different filesystems, upgrades, behind firewalls or weird networking, interacting with email providers and their particularities...