Domain mapping does not redirect to mapped domains

I created a subdomain (www.maindomain.com/subdomain) and mapped four domain names to that subdomain (basically the subdirectory is the ".de" version of the ".com" maindomain, that is "maindomain.de").

Anyhow, the mapping does not redirect: No matter what URL I enter (any of the mapped domains, "maindomain.de" or even "maindomain.com/subdomain") - I always end up on "maindomain.com", the main domain of the multisite...

The only way to get a glimpse of the subdomain's frontend is when I go to the design section of the backend and choose to "customize" the theme. Any other try to access the frontend ends on "maindomain.com"...

Thanks for ideas how to solve this.