Bejamin, I believe you want a Backup MX server which leverages a free license of SmarterMail to queue mail for your domains while the primary is offline. Once set up, you just need to have two MX records with the backup MX having a lower priority. This ensures mail isn't sent to it unless the primary MX record is offline or unreachable. Hope this helps!
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com