[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.