Hi,
Actually this is what happen. We have an email server hosting different domain name on the same server. One of our customers ask if possible that they could make their email domain(currently hosted on our server) as their backup mail server because they have plans to put a Primary Email Server on their office. All accounts are still in the on the current server. What we did is to reconfigure the DNS server and pointing MX record with priority 10 to the new Primary Email Server IP address and Slide the current Email Server to priority 20. With this they were able to collect mails when there new email server are down. I don't know what implications on our server with this kind of setup. My only concern is if spam filtering still functions this way.
Regards,
Rocel