a. no touch the default, but disable the client one
b. a new IP for app & relay: one receive one for app+IP Arrange with no-relay , and another for relay
c. F5 to balance the new IPs
Exchange 2010 Connectors – briefing
How To Allow Relaying in Exchange 2010 and Exchange 2007
Exchange Server 2007′s Receive Connectors have a different view of IP binding, unlike Exchange Server 2003/2000’s SMTP Virtual Servers. A binding is considered unique if it uses a unique combination of IP Address + TCP port + RemoteIPRanges. So it in fact allows multiple Receive Connectors to be created using the same IP address + TCP port, as long as you have different IP ranges specified.
Prevent registration of multiple IP addresses in DNS