Inline load balancing

You need a forwarding virtual server if your backend servers initiate new connections out their default gateway(the F5) The F5 isn't a router, it needs a VS that maps to a pool containing your upstream router to handle the requests.

Being these are mail servers and probably deliver mail outside of your network as well as probably grab updates/signatures, I would assume you need this. There are other implementations though where the backend servers only receive traffic through the default gateway, so a forwarding virtual isn't necessary.

A real-world example I have is a VPN gateway that only accepts connections, it learns routes to internal networks via other interfaces, gets managed/updated through a management interface, and never needs to initiate a new connection outbound to the internet via its default gateway. No forwarding vs was needed in this implementation.

The way I have implemented this is as follows.

I have an external interface, this is the interface/subnet that my virtual server IP addresses are on. I have an internal interface, this is the interface/subnet that the backend virtual servers are on. There is a router on the external interface, which is the default gateway for the F5.

On the internal interface, I have a virtual server, with a destination network of 0.0.0.0 mask 0.0.0.0. It is a Performance Layer 4 type VS, for all ports and all protocols. The default pool for this VS contains the IP address of the router that sits infront of the external interface. My backend servers then use the floating self-ip as their default gateway.

There are other ways to accomplish this as well. See http://support.f5.com/kb/en-us/solutions/public/7000/500/sol7595.html for more info. You can setup a forwarding server that does use the routing table of the F5 to determine the next hop.