This could be a SPF (Sender Policy Framework) problem. When you use a SPF enabled domain to send to your server and your server forwards this to gmail, gmail may just silently drop that mail. There will be no error message from the server and you will not have it in your Spam folder.
I really hate the whole SPF concept as it has many design flaws and introduces a lot of problems, but unfortunately many of the big providers use it.
As SPF breaks SMTP forwarding, you need to implement SRS. The only way I could get it running was using postsrsd (https://github.com/roehling/postsrsd).
You are absolutly right!!!
I changed the forwared email adress to a non gmail adress and voilà the email was sent correct to both mail adresses....
@MuhKuh I have to compile postsrsd like github readme shows in order to get this working with i-mscp?