If, like me, you find rules engines a bit intimidating you might get good mileage from regular expressions. You could build a set of expressions paired with forwarding addresses. If pair.expression matches the subject line, forward to pair.address. Routing like this is much easier if you can make customers send you mail via forms where you can control the content of the subject line! I spent some time last year reviewing commercial products that make their living doing this stuff and some have pretty sophisticated rules fer sure.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi