We have an Exchange 2010 server as our primary mail server. We want to put in a backup email server in case our Exchange server goes down. I've set up IMAP servers before with Debian, so I was thinking about setting one up. The idea would be to hold all the emails destined for the Exchange 2010 server on the Debian server while the Exchange 2010 server is down. However, when the Exchange 2010 server comes back up, the Debian server would send all those emails it had been queuing up to the Exchange server for delivery.
Not quite sure how to set the Debian server up like that. Any suggestions?