We have 2 exchange servers. A 2003 server in our UK site and a 2007 server in our china site. The China domain is a child domain of the UK domain. The two sites are connected via a VPN.
Email between users in each site is being sent and recieved directly over the VPN between the servers using a routing group connector. The UK users have a .co.uk email address and the China users have a .CN email with the external MX records of each pointing to the UK or China server respectively on different public IP addresses. Each site has its own SMTP connector to the local ISP.
What I would like to do is route messages of a certain size out over the internet instead of directly over the VPN. So if someone sends say a text only email then it will go direct. if they send an attachment then it could go out to the internet and back in. Is this possible?