I want to setup a special SMTP environment: two different SMTP servers that should be transparently accessed from SMTP clients.
Let's say I have a RFC compliant Postfix running at 192.168.0.1:25 and a RFC ignoring server fooling clients at 192.168.0.1:2525.
Now I want the following. Most connections should be handled by Postfix as it is listening on the correct port. But with different iptables rules I currently REJECT/DROP connections due to RBL listings, abusive behavior or exceeding limits; just to reduce the load on the Postfix server. Now I no longer want to DROP them, but instead forward the connections to port 2525. The second server is to act as a tarpit and then defer/reject the mails.
I can't figure out how to forward connections depending on other iptables rules that are in the INPUT chain. There I use xt_recent and limit filters to dynamically decide between ACCEPT and DROP.