Remember that when you change something in your ispconfig web admin, that file will be overwritten and forwarding for that domain will stop again .. what you need to do is set the mailserver to "external"
1. login to the web admin
2. search your domain
3. Goto tab: Options -> Mailserver -> External Mailserver
4. Goto tab: Co-domains -> Click on the domain -> Options -> Mailserver -> External Mailserver
Now ISPConfig knows that all mail for @mydomain.com is not hosted on his server, but will send it to the specified one in your MX records
(These setups are typical when you only host websites on your server and use a different one for mail)
(Deleting this thread wouldn't help other people that might run into this problem, replying on your own thread with a found solution is what makes a forum like this so helpfull)