I am trying to setup MTA which should submit different intranet mail gateways based on the recipient's domain. For that I thinks transport_maps in postfix should be setup for selective domains; if recipient domain is not matched with any line in transport_maps than it will do default delivery which is smtp. What I am confused is whether to use relay or use smtp as transport types and what difference will it make.
Based on received answers one difference is MX priortization, but I turn off mx discovery by enclosing ip/domain in big brackets, will both of them be same or is there any other difference which I am missing.