We have a private cloud with all of the servers sending mail via mandrill, and each app/server has its own key. We have a few legacy apps, for example a .net 1.1 app, that can’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. I found how to use a different smart relay for different destination domains, but not for different sending servers. In other words, one server sending to gmail vs yahoo can use a different smart host. But two servers sending to gmail seem to want to use the same key, and I need them to use the key that matches the sending server. Is this possible?