[exim] exim relay backup question in case of disaster?

I have an interesting request from a company that we are relaying
mail for. We are simply doing virus/spam scanning and then
passing the mail onto their mail server in another location.
If there is a disaster, they want access to the mail that is
queued up on our Exim relay server.

Hmmm....
They get about 3,000 emails per day, so if there are 5,000 + emails
in queue on our server after a disaster at that company, I would
think that we could make the changes to Exim to be the final destination
and then offer a webmail interface. They have about 120 users, but
management would be the only ones to need the emergency mail access.

I am thinking this is more complicated than it seems.

What do you all think?
What would you recommend?

Andy