Normally, if you want mail for the domain to go to the parent, you wouldn’t enable mail for the alias (i.e. when you created it, it sounds like you chose “Alias, with own email”).
So, if you want emails for the alias domain to go to the same user in the parent, just make an alias, don’t make an alias “with own email”, because that would mean you’d have to manually create all the aliases to redirect mail to your parent domain.
So, delete the alias with own email, then create a new one without own email, and check the box for “Accept mail for domain”.
Ok, I don’t remember which kind this server was, so I followed your advice and removed the alias-server.
Then I’ve created a new one via Button " Alias from mydomain.com " and leave “Accept mail for domain” unchecked (or do I need to check it)?
Still don’t get any mail that go to the .de domain. @Joe Do I need to activate or deactivate the Email-Checkbox in the Server-Settings of “Alias from”-Server?
Yes, I’ve seen it, and caused of this (and because I don’t remember how to setup this server in the past) I fully removed this Alias-Server and set it up it again, as “Alias from” without Email.