The part that I especially don't understand is that there is supposed to be some kind of redirection from mydomain.de to mydomain.de/index.htm which is not acccording to their guidelines. I don't have any redirections on that site.
Don't take the slightest bit of notice of that email. It is highly unlikely that google sent you that email. It sounds 99.99% fake and from a spammer of some sort, most probably from a clown.