i operate my own IMAP server (192.168.1.20) on an static IP (213.135.x.x). Access from outside works fine. But access from inside the network to the IMAP Server is not possible.
Does anybody has an idea from access from inside the network?
you have to create DMZ (put your server in another network due to NAT problem, or, you can set router with static DNS entry that will resolve to your local address from your local network. And use router as your DNS resolver.
More about NAT problem you have:
when you connect to router from same network, you still attempt to connect to external IP and in one moment server and client starts to communicate directly while they expect packets to come from different source.