Postfix: emails addressed to multiple smart hosts

Hi,

I am in process of setting up a SMTP server on RHEL 6 using Postfix.

I am stuck at one point so any help would be appreciated.

I have configured my xsender to use this new SMTP server as its mail server.

If I send any emails addressed TO example.com the SMTP server should use the DNS MX record and forward that email.

If I send any email to any domain APART from example.com the SMTP server should use "myrelayhost.com" as its Relay Host which is an Iron port device.

Basically I do not want any email going internally to organization to hit my Ironport Device "myrelayhost.com"

How is this possible using postfix?