Look into Dell EMS.http://content.dell.com/us/en/enterpris ... management
It's basically a clooooud based disaster recovery and archiving product. We looked into setting up a second Exchange 2010 server running on Win2K8 R2 and the cost was prohibitive. Dell EMS also took care of archival needs for any legal requirements we may have. We have an office of around 50 people.
You can flip a switch whenever your email server dies and your employees can log into Dell EMS's website for web mail. It even has an Outlook plugin that automagically redirects stuff to the Dell servers when it detects that you're in "backup" mode. And when you go back to normal, you can sync mail back to your main server.
There is a recurring fee per year, of course, but I still think it's worth looking into with your scenario.
We have a completely different physical location (data center) that we could use to host another Exchange server, but that means more maintenance on our end, managing another server, and paying for bandwidth at the data center. Having our backup offsite is excellent and not having to manage it is icing on the datacake.