[Skunkworks] queuing secondary domain mails

Am using Zimbra on a server which works well on my main domain. I have other domains where secondary preferred pref points to this server. What I want to achieve is, the main server to be queueing mails for other domains if the first preferred server is down or unreachable. Zimbra works together with postfix. Linux Guru`s pls assist. Edwin 254 722 841853