I've recently been in a situation where an Exchange email server went down and we weren't able to bring it back online after 48hrs, so some incoming emails bounced. Also, users weren't able to send or receive emails for a few days, which was a sore point and caused some early withdrawl symptoms.
In a post-mortum someone came up with the idea of setting up a quick and dirty webmail server to catch those emails during the outage so that at the very least people could receive and send emails. I've never done this, but it sounds like a good idea. Has anyone tried something like this before successfully?
What I usually do is set up a mail forwarder as a backup MX - that is, a mail server that isn't the destination for any emails but can hold on to them (leave them on hold) in case something goes wrong. The upside to this is that the moment your main mail server goes back up the emails start being delivered as if nothing happened - you don't have multiple inboxes or other complexities to manage. The downside, of course, is that in the meantime the emails aren't accessible to the end users.
I'm a GNU/Linux admin so I usually use Postfix for this - it's light and it gets the job done, no need for anything fancy as the task in hand is really quite simple. You can use this link as a guide. But I'm sure there are plenty of Windows alternatives if you prefer to stick to that.
Hand this responsibility over to somebody else. Postini, AppRiver, and Symantec are all examples of companies that do this for you. Let them filter your spam before it gets to your network and spool your email should your server go down. It's much better than putting together your own solution which could also go down (like if you have an extended internet outage).
Better yet, move to hosted Exchange. I can't imagine that it would be a difficult sale to the bosses following an outage that lasted over 2 days.