How to configure Postfix as a SMTP gateway?

■ Requirement: Configure postfix as a smtp gateway server
■OS Environment : Linux [RHEL 5, RHEL 6]
■Application: postfix
■ Assumption : 

•       Domain name= example.com, 
•       Internal Mail server IP = 192.168.1.3, 
•       Gateway mail server IP = 192.168.1.2, 
•       Internal postfix smtp is pre-configured. 

■Implementation Steps :

A. DMZ Mail Server Setup (or gateway mail server):  The DMZ mail server forwards the inbound mail to the internal mail server and delivers the outbound mail to internet.

1. Edit /etc/postfix/main.cf and update the lines below.

mydestination =
local_recipient_maps =
local_transport = error:local mail delivery is disabled

mynetworks = 127.0.0.0/8 192.168.1.3
relay_domains = example.com
transport_maps = hash:/etc/postfix/transport
smtpd_recipient_restrictions = permit_mynetworks
reject_unauth_destination

2. Edit the file /etc/postfix/transport and add the line below.

example.com :[192.168.1.3]

NOTE : If you would like to use multiple internal server for multiple users/domains then postfix transport maps should be changed.

e.g. in main.cf

transport_maps = /etc/postfix/transport

in /etc/postfix/transport:

user1/domain1 smtp:1-mailserver.example.com
user2/domain2 smtp:2-mailserver.example.com

3. Execute following commands to reflect above :

$ postmap /etc/postfix/transport 
$ postfix reload
NOTE : That will deliver email for user1 and user2 to [1,2]-mailserver.example.com. If you don't specify anything in the transport map, the default transport (which is usually deliver locally) will be used.

B. Configure Internal Mail Server :

The internal mail server holds the mailbox and forward all outbound mail to the DMZ mail server.

1. Edit /etc/postfix/main.cf and update the lines below : 

transport_maps = hash:/etc/postfix/transport

2. Edit file /etc/postfix/transport and add the lines below :

example.com :
.example.com :
* smtp:[192.168.3.2]

3. Create a transport database file :

$ postmap /etc/postfix/transport

4. Restart the Postfix : 

$ service postfix restart