People usually help with problems they have had and they resolved.
So if no one suffered this issue, maybe no one helps.
IMHO it seems that the system is not forwarding on GREEN.
You have a rule on RED to port forward some traffic to an internal server.
Let's name mail.mydomain.com as 88.125.36.27. If you reach your firewall from RED, the port forwarding rule is fired, and the traffic is routed to the internal IP of your mail server.
BUT, from inside the IP 88.125.36.27 is for the firewall, and if you try to use
http://88.125.36.27 the port forwarding rule isn't fired (as it is a rule for RED only).
Your possible options:
1- Change your internal DNS resolution to point mail.mydomain.com to the internal IP.
Go to Network->Edit hosts. Create a new host:
IP: <<IP of the internal server>>
hostname: mail
domain: mydomain.com
Apply. Test
nslookup mail.mydomain.com to see if resolves to the internal IP
2- Create an static route to your mailserver (not sure if it works).