[exim] Linux-HA / Routing according to IP address received u…

I've setup a Linux-HA cluster using Heartbeat and I'm looking at the
best way to setup a highly available SMTP service. I've got two ideas.

1. Have a single MX record that points to the IP address held by the
active node in the cluster. This copes with hardware failure fine but
doesn't fail gracefully if there's a problem with exim.

2. Setup each node as as primary and secondary MX hosts and have them
store and forward the messages onto the IP address of the active node.
I'd prefer this solution but what do I need to do to route mail
according to the IP address it was received upon? I want to be able to
forward the message to the shared IP address if it's not received on it.

If there's a third, obvious and sensible solution I'm missing - please
let me know :-)

Many thanks,

James