There are a fair few ways to do this, like port based routing, but as this is email, i'd think the simplest way would be to use an isp smtp relay to send all mail via. You can just tell postfix to send ALL emails to a single external server, a service that most isp's will provide for various reasons like spam protection, which is fully available for their own customers to use. you just need to then route the IP address of that server through the interface you wish to use and you're away, and it fits in easily with basic networking, and means you've no need to do anything fancier.
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