Here's what I try to accomplish:

I want my iRedmail server to receive mail for several domains. The email for one (1) of these domains should be re-routed to another email server and then also be able to act as a smart relay for that email server and that domain. 

Incoming email:

Internet -> iRedmail -> Postfix on other server -> local mailbox

Outgoing email:

User writes email -> Postfix on other server -> iRedmail -> Internet

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I have been trying to get this to work for many hours now.   What I have tried so far:

Created /etc/postfix/transport and entered the following info into it:

domain.tld  relay:[server.hostname.tld]:port

I then ran postmap /etc/postfix/transport

I then added that file to /etc/postfix/main.cf:

transport_maps =
    hash:/etc/postfix/transport

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This works if I send email locally on the iRedmail server.  If I send an email from Gmail or something I get "Relay access denied" from Postfix on my iRedmail server.  I can send email from the other server to a user on the iRedmail server without any problems. If I try to send email from the other server to Gmail or something, I get "relay denied".

Any ideas?   

Thanks.