(Zimbra Community 5.0.8 & NE 4.5)
Had a hard time finding this info and finally found one post that correctly pointed in the right direct but didn't actually detail all the info.
Backup MX: Have Zimbra accept mail on behalf of some other domain in the event the other domain is temporarily unavailable and forward to other domain when it becomes available.
Yes I know about dynamic dns providers offering backup facilities but I want my mail housed/processed by people I know and trust and not some other company. I also don't want to pay extra to some other company when we have perfectly good facilities ourselves.
Postfix does this with the "relay_domains" functionality. Although Postfix is the MTA under the covers, I want a solution that will work with Zimbra's configuratinon umbrella and not get smoked by an upgrade or system restart.
Server "thisdomain.com" wants to provide relay services for server "thatdomain.com".
1. Add the domain to the server:
zmprov cd thatdomain.com zimbraAuthMech zimbra
2. Create full-domain forwarding to the intended destination:
zmprov md thatdomain.com zimbraMailCatchAllAddress @thatdomain.com
zmprov md thatdomain.com zimbraMailCatchAllForwardingAddress @thatdomain.com
zmprov md thatdomain.com zimbraMailTransport smtp:thatdomain.com
3. Have "thatdomain.com" mgnt add "thisdomain.com" as a higher number MX record to their DNS entry.