We have a medium sized cloud with 90% of servers sending via mandrill and each app/server having its own key. We have a few legacy apps, for example a .net 1.1 app, that won't send via TLS over 587, and mandrill doesn't support plan smtp over 25, so we set up a postfix server to do relay to mandrill via sasl authentication and a single mandrill key. I found how to use a different smart relay for different destination domains, but not for different sending servers. We use Mandrill to separate outgoing email by application, so 2 users both with @gmail.com could still go out over different mandrill keys if they are customers on 2 different platforms. If this is possible, can someone please point me toward some documentation?