Dns round robin: I could try.. but we use internal ips. It is still possible though. But i dont know how good it works if a node goes down. maybe we user the loadbalancer for this. Mailcleaner had the option to do this. But anyway we will try a different approach.
So it would be the default relay ? And if we want to user extern Mailservers on local sites we go with transports right?
Not quite sure if I understand the question completely - if not please explain some more
but you use transports to override the 'default relay' for certain relay domains.
so if you set your default relay to main.server.com, and have 2 domains 'a.com' and 'b.com' configured as relay domains:
* if you create a transport entry for b.com pointing to second.server.com - mails for firstname.lastname@example.org would get sent to second.server.com
* mails to email@example.com would get sent to main.server.com (since it's the default relay)