[Configuring Postfix as an Inbound Mail Gateway by Tyrone Cusi]

Here are the details, you can use this example:

In this scenario your Postfix server (gw.example.com) is relaying mails for hr.example.com and sales.example.com. Each domain has its own internal mail server.

1. Make sure that the DNS has been configured correctly with MX records for hr.example.com and sales.example.com pointing to the gateway gw.example.com.

2. In your main.cf file, set relay_domains to include the two internal domains:

relay_domains = hr.example.com, sales.example.com

3. Make sure that the transport_maps parameter points to your transport lookup table:

transport_maps = hash:/etc/postfix/transport

4. Add entries to your transport file for each domain pointing to the correct internal mail systems:

#
# transport maps
#
hr.example.com relay:[mail1.example.com]
sales.example.com relay:[mail2.example.com]

We've used brackets around the internal mail system host names to disable MX lookups for those systems.

5. Reload Postfix so that it recognizes the changes in its configuration files:

# postfix reload

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