Help: redundant SMTP relay

Folks, I need your knowledge on this one coz I'm far to be familiar with SMTP servers.

I have 3 mail server (exchange) and I'd like to implement a redundant path for relaying mails.
Here comes the topology:

Code:

---

Server A--------------------------------------ServerB - domain.com
    |                                        |
    |                                        |
    |                                        |
Server C--------------------------------------|

---

The network that links serverA & serverB is high speed.
The network that links serverA to ServerB through C is low bandwidth.

What I would like to do is to configure a backup SMTP route to serverB smth like
domain.com -> Primary ServerB
domain.com -> Backup ServerC

So when ServerB is not IP reachable from ServerA I would like that the serverA relays frames through serverC. (ouf!)

Thanx guys