We have multiple public IPs from two diff carriers (comcast & at&t). All of those IPs come into the same building and terminate into the same router. So, my router has two WAN ports. WAN1 has the comcast IPs and WAN2 has the at&t IPs.
I have MX records set up on metrics of 10 and 40. MX1 points to the comcast side. MX2 points to the at&t side.
In my router, I have rules listening on each WAN port and sending port 25 traffic to me single exchange server (exchange 2010). My hub transport is listening on both IPs.
I'm thinking that with this setup, if WAN1 fails, mail will still flow.