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.