[kernx16]

That's going to take a lot of processing power to be able to handle their amount of customers on a single domain. Even loadbalanced behind the domain to multiple servers, even running kafka, probably could bottleneck it through a single domain. Regardless, even if you did A.com, you have to redirect the customer to whatever actual link they originally wanted to go too from MailChimp's server, instead of the customers server.

[Beldin]

Sorry for not being clear. My idea was a small file on A.com/mailchimp that takes the arguments and passes them on to mailchimp's servers (possibly already spreading, e.g. mailing1.a.com.mailchimp.com/clicktrack?click=...) This would redirect to A.com's targeted link.

The processing power would still be needed on mailchimp's side. All the customer (A.com) does, is add a tiny redirecting script on their site.

It's similar to how one fingerprinter for android works.Get your client to forward you the user details. User only sees the client (so nothing suspicious, no 3rd party to be blocked), while encryption ensures that your client has to forward you the data - they cannot parse it themselves.

[mpeg]

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.