I have the following problem ...

I have configured two domains (domain1.com, domain2.com), two users with mail-adresses to their domains, e.g. user1@domain1.com and user2@domain2.com and two subdomains for login (mail.domain1.com and mail.domain2.com).

The problem now: I can login with both users using both subdomains.
It means I can use user1@domain1.com and login at the loginscreen of mail.domain2.com - this I donīt want.

I want that the users can only login at their URL (user1@domain1.com --> mail.domain1.com and user2@domain2.com --> mail.domain2.com).

Whatīs wrong there?