Having an inbox does not mean using it or checking it regularly. Think of all the old people with tablets who don't even know what their email address is. Or folks like me who have several. What I'm saying is that "an email address exists, somebody must be checking it" is a flawed argument.
With over 2.6 billion active users and over 4.6 billion email accounts in operation, email is the most important and widely used communications medium on the internet.
In fact, this article: https://phrasee.co/blog/a-brief-history-of-email/
says the following:
Still, 2.6B is a lot. So there's that.