We would like to implement a backup MX for one of our client's Zimbra servers.
The backup MX would be a non-Zimbra Postfix server.
When we have built backup Postfix MX servers for Exchange boxes, we use a built-in Microsoft utility to extract a list of valid email addresses on the Exchange box (mailboxes, aliases, distribution lists, etc.)
We use this list to build the relay recipients map on the Postfix backup MX, so that the backup MX accepts email only for valid recipients on the Exchange server and eliminates the possibility of backscatter. No Starch Press's The Book of Postfix has a section that shows how to automate the entire process on both the Exchange and the Postfix servers.
We would like to do the same thing with this client's Zimbra server. Has anyone done this before?
Alternatively, we understand Zimbra internally just does "store and forward" on their backup MX server(s). If so, can a Zimbra person explain how you solve the backscatter problem please? (i.e. when the backup MX forwards email to a non-existent email address on the Zimbra server and the Zimbra server sends the backup MX server a bounce message, which the backup MX server would then try to send on to the original sender.)