It seems, that
htm176.htm-server.de
does not allow forwarding of emails of a different domain as a mail-relay.
As you can see, neither a MX-record nor an A-record of htm176.htm-server.de does exist in the DNS. So mails will be rejected. It is a security-feature.
In the opposite Strato allows it, because they offer the mail-relay.
If htm-server.de also allows to be handled as mail-relay, you should have a look on their support pages, how to enable the relaying.