Google doesn't have a monopoly on email. Check your (personal) inbox - I'd bet the majority of emails on the first page are not from Gmail users but rather from various companies and organizations. Even among individuals, while many people get their email from Google, the most popular client (which is what's relevant in this case) is the mail app on the iPhone. Microsoft and Yahoo still have decent market share as well, though much less than I thought before I started writing this with 12-18% between them according to my not-super-rigorous Google search.