Email routing problem, async routing

A full mesh network is generally considered a good thing.  It means that all routers have a direct connection to each other and that there are multiple routes for each node.  However, in today's world with statefull routers and firewalls, this causes problems for email routing.  This is due to TCP's three way handshake and "established" connections and statefull firewalls.
 
When a mailserver opens a connection, it knows the IP and port number that it is supposed to talk to.  The statefull firewalls and routers/switches also note this connection so that they know what is going on.

 
 
The problem with async routing shows up here.  The main firewall and router only "see" the out bound connections, and nothing knows about the inbound connections coming back thru the secondary connection.