I'm having a very similar error as this thread.
let me start out by saying the forwards work, I setup a seperate server with only a standalone mail server running to test router and network config and it worked fine. However I'd really like to use ISPconfig for everything.
Here's the general email I get back after trying to use another external address to send to this machine.
This is the Postfix program at host nickel.vcn.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to <postmaster>
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The Postfix program
<email@example.com>: host 126.96.36.199[188.8.131.52] said: 554
<firstname.lastname@example.org>: Relay access denied (in reply to RCPT TO command)
My forwards are setup, my domain is forwarded correctly, and firewall's do not seem to be blocking anything. I use debian 3.1/sarge. I used the perfect setup guide and followed every single step to the letter in setting my machine up from a fresh format to it running now.
I am finding nothing in the documentation thus far that's helped so if anyone could shed some light that would be great.