Ok, I have yet to find a definitive answer for this. I have an exchange server, and am trying to find a good fallover solution. If the power goes out what is the best way for me to hold my email until the server comes back onine?
If the power outage is short, you don't need to do anything. Other mail servers will simply retry again later. If you need to survive days-long outages, then you can use a third-party MX host to buffer your incoming email.
Use a backup e-mail service.
Your MX records have a priority, if your primary MX server is unreachable, senders will try to deliver on the next one. Simply add a backup service with a low priority MX record, they will keep your e-mail for you till your own server is back online.
It's possible to do this with a second exchange server if you want to do everything yourself.
Google for "Backup e-mail service", you'll find lots of them. One example is Dyn e-mail backup.