Postfix Issues - 2 Servers, 1 Router

[3DPeruna]

I have two servers that sit behind a Cisco router. I'm having problems sending email between them. Both can be seen from the internet and both can send/receive email to domains that aren't on either server. But neither can send email to an address on the other server.

Both are running ISPConfig 3, too.

Server1 has an external IP address. Server2 has an internal IP address that uses a NAT to pass it through to it's static IP.

When mail is sent from Server1 to Server2 (or vice versa), I get a address@domain.com mail for domain.com loops back to myself error.

Ideas?