It's really not a problem to handle any number of users on a single domain, you can load balance, serve from different geos, etc. Case in point: google.com Th

It's really not a problem to handle any number of users on a single domain, you can load balance, serve from different geos, etc. Case in point: google.com The real reason they don't do this is because it's a more involved set-up from the customer's side. As people mentioned, enterprise-oriented mail services will do this kind of thing but MailChimp is a long tail solution.